Hmmm... could be that Slughorn's thinking of someone but I'll bet you ten Galleons that R.A.B is Regulus Black, Sirius's little Death-Eater brother.
Couldn't be anybody else. All the facts fit, including Sirius' comment that "he got in so far, and found out that you couldn't back out."
What it tells me is, Harry's going to have to go back to No. 12 Grimmauld Place in Book 7.