Recently I have been thinking that perhaps RAB could be Borgin of Borgin and Burkes. I found this theory on Mugglenet that pretty much sums up exactly what Iv'e been thinking.. (I know its long bear with me).
In my editorial I intend to crush the 'Regulus Black Theory' concerning R.A.B. In order to do this, I must first return to the source - the one artifact R.A.B. has left us as a clue to his identity - his note to the Dark Lord:
To the Dark Lord
I know I will be dead long before you read this
but I want you to know that it was I who discovered your secret.
I have stolen the real horcrux and intend to destroy it as soon as I can.
I face death in the hope that when you meet your match,
you will be mortal once more.
R.A.B.
(609, HBP)
We can deduce from this note that R.A.B. knows Voldemort personally, as he expects Voldemort to recognize him just by his initials. Additionally, we can see that he expects Voldemort to be shocked and/or impressed that R.A.B. has stolen his horcrux because he seems very intent on receiving the credit for "discovering" his secret. I do not think however we can prove that R.A.B. is dead since Voldemort has not yet read the note.
Now when we think of someone knowing Voldemort personally, we automatically think of Death Eaters, but I think that the note taken in context of this next quotation proves that R.A.B. could not be a Death Eater and thus not Regulus Black.
"'And then I ask myself, but how could they have believed I would not rise again? They, who knew the steps I took, long ago, to guard myself against mortal death? They, who had seen proofs of the immensity of my power in the times when I was mightier than any wizard living?'"
- Lord Voldemort
(648, GoF) (underlining is my emphasis)
Here Voldemort reveals that he has told his Death Eaters of how he plans to achieve immortality through his horcruxes and that he probably told them "long ago." Now if R.A.B. is a Death Eater, why would he brag that he "discovered" Voldemort's secret. If he were a Death Eater, he would have been told directly by Voldemort about his horcruxes. Therefore, since R.A.B. seems very pleased with himself for "discovering" this secret, he cannot be a Death Eater and thus not Regulus Black as the DE's already knew Voldemort's secret.
Some might say, however, that R.A.B. simply discovered the secret location of Voldemort's horcrux and could, therefore, still be a Death Eater. But I think the phrase "discovered your secret" suggests the big secret of Voldemort's plan for immortality and not just the certain location of one horcrux. In conclusion, all evidence that can be collected from the books concerning R.A.B. and Voldemort's horcruxes proves that R.A.B. cannot be a Death Eater and not Regulus.
In addition to clues from the books, J.K. Rowling herself hints that R.A.B. is not Regulus Black in her recent interview with Emerson of MuggleNet and Melissa of The Leaky Cauldron.
MA: R.A.B.
JKR: Ohhh, good.
[All laugh.] JKR: No, I'm glad! Yes?
MA: Can we figure out who he is, from what we know so far?
[Note: JKR has adopted slightly evil look here] JKR: Do you have a theory?
MA: We've come up with Regulus Black.
JKR: Have you now?
MA: Uh-oh.
[Laughter.]
JKR: Well, I think that would be, um, a fine guess.
Emerson and Melissa directly propose the 'Regulus Black Theory' to J.K. Rowling in this excerpt and she replies first with a sarcastic "Have you now?" that makes Melissa say "uh-oh" and then with "Well, I think that would be, um, a fine guess" and I can physically see the sarcasm exuding from J.K. Rowling's "um." This seems to tell me that J.K. Rowling is telling us we are on the wrong track with the 'Regulus Black Theory.' However, she does call it "a fine guess" (no matter how sarcastically she says it) which tells me she saw the 'Regulus Black Theory' coming from a mile away, but more on that later.
Finally, the 'Regulus Black Theory' destroys the interesting foil between Regulus and his brother Sirius. If Regulus is R.A.B. it would suggest that there was something intrinsically noble in the House of Black as both its' sons would have died sacrificing themselves for the good of others. But I think that J.K. Rowling has chosen to once again stress her theme of choices by showing both Sirius and Regulus coming from the exact same family situation, but Sirius dying nobly in order to save Harry and his fellow students, while Regulus dies cowering as he is to afraid to act for Voldemort and to afraid to act against him as R.A.B.
Finally, I found it. I found a character that would have known Voldemort personally but is not a Death Eater, has a knowledge of the Dark Arts and would know about horcruxes, even has a specialty in enchanted objects (especially dark ones), and would have had reason to suspect Riddle's dark future before he became Lord Voldemort. I am talking not of Voldemort's old boss, Caractacus Burke, but his oily business partner Mr. Borgin.
Now, we have met Mr. Borgin twice so far in this series (CoS and HBP), and two interesting features have characterized these two scenes. First, simply, we have never learned his first name, as both Lucius and Draco Malfoy both call him either Mr. Borgin or just Borgin; therefore, he is not eliminated from the R.A.B. mystery. Additionally, in both cases we see that he detests and despises the Death Eaters and their methods despite his own interest in the Dark Arts. In CoS after Lucius Malfoy leaves the shop, Borgin makes a comment about Malfoy not selling him half of what is in his manor and then retires to the back of the shop muttering darkly. Then in HBP, when Malfoy enters the shop, it is said that Borgin "was wearing a curious expression of mingled resentment and fear." Malfoy threatens him and shows him something (probably the dark mark) and Borgin is said to be "looking very frightened." In both incidents where we meet Borgin, he is shown to hate the Death Eaters/Malfoys, which shows us that he has plenty of motive to do the things that R.A.B. has done to try and thwart Voldemort's plans for immortality.
Additionally, as I stated earlier Borgin would have suspected Riddle's dark future before he openly became Lord Voldemort, giving him a head start on deciphering Voldemort's secrets and discovering the horcrux and leaving the R.A.B. note. He would have suspected Riddle's dark future simply because of Hepzibah Smith. Surely Borgin would have noticed that Hepzibah Smith was murdered two days after Riddle's visit (a visit Borgin would have, of course, known of), that Riddle disappeared without any explanation after the murder and never returned, and that Mrs. Smith's two most precious artifacts were eventually proven to be missing and stolen.
So... Borgin has been shown to hate Death Eaters, and, by association, Voldemort, he would have known Voldemort personally, he has a knowledge of the Dark Arts and Dark objects that would give him a perfect base of knowledge from which to unravel Voldemort's secret, and he would have suspected Voldemort's intentions early in life thereby giving him a head start and allowing him to beat even Albus Dumbledore to the horcrux. We have drawn up a detailed character analysis of Borgin and it seems to fit perfectly with the mysterious R.A.B. So now let us look at some contextual clues that J.K. Rowling has left us that point to Mr. Borgin.
First, Regulus Black. To begin with, he has two of the proper R.A.B. initials immediately calling attention to him. Not only that, but we know that he had a relationship with Voldemort that did not end on a happy note. All things we can expect of the real R.A.B. But some things just don't quite fit. R.A.B. we can see is extremely clever and a man of action but we have learned from Sirius that Regulus is most likely weak and cowardly. Additionally, the Death Eater connection just doesn't quite work. How could Regulus discover the secret if he already knew it? So everything about Regulus is close, but doesn't quite work.
In other words, he is the perfect red herring. The reader's attention is immediately drawn to Regulus Black (because of the R and B). Then, upon closer inspection, the reader is rewarded once more. Regulus knew Voldemort personally and this did not work out too well for him. While delving deeper, however, you begin to find small inconsistencies with the 'Regulus Black Theory,' but they are small enough for someone to ignore as the power of the two initials' connection overshadows them. But if you accept these inconsistencies as inconsistencies the Regulus Black morphs from the perfect red herring into a huge clue that R.A.B. must be someone else.
Next, Caractacus Burke. In both instances where Harry has encountered the Dark Arts shop, Borgin and Burke's, Borgin has been running the shop. We get a good feel of Borgin's personality and what exactly makes him tick, unlike the always-absent Burke. Despite all this, it is Burke's first name that Dumbledore casually drops in conversation. Why does J.K. Rowling gives us the first name of a meaningless character, making his initials C.B. and effectively eliminating him from the R.A.B. mystery? Why bother to eliminate him at all? Nobody would've suspected him in the first place. Sure he has the right last initial, but why would we waste our time suspecting him? We don't even know him. Unless J.K. Rowling must eliminate him from the mystery in order to leave a clear path to his oily business partner Mr. Borgin as the real R.A.B. In conclusion, once again J.K. Rowling has shown that she does not often drop names without a purpose and that meaningless characters can become huge clues to help solve the mysteries of this series.
If you are still reading this I would like to congratulate you on your diligence and perseverance in pushing through my research paper of an editorial. And I would like to reward you with the fact that the true identity of R.A.B. is none other than Mr. Borgin of Borgin and Burke's.
I'll tell them that you posted it! 
! I know your just trying to come up with more opinions, that's what I keep doing too. I just really hate that Regulus Black Thoery! 