I have come to the conclusion that R.A.B. is definitely Sirius's younger brother Regulus Black. Also, he had help from Kreacher to get the locket. If you read carefully in OoP it says an item they found at Grimmauld Place was a "heavy locket that none of them could open" and in HBP Merope's locket was described as "heavy". Kreacher would have had to obey Regulus, as he is a member of the Black family and Kreacher obeys the Blacks. It must be Kreacher and now Kreacher has the locket - he kept trying to sneak things out whilst they were cleaning the house in OoP. He was most likely sworn to secrecy by Regulus (whose middle name I believe to be Alphard, after his Uncle that is also mentioned in OoP). Kreacher would have been able to get on the boat in the cave because he is a house elf and Voldemort wouldn't deem them powerful enough to get that far, or magical enough either - just like under-age wizards. Regulus would also have referred to Voldemort as the "Dark Lord" because he was a Death Eater. Also, Regulus has been dead 16 years now and in the note within the locket it says the writer will long be dead.

Please let me know what you think of my theory!