According to Nimbus.com, Isabel Nunez, the Portuguese translator for the Harry Potter books, stated that J.K. Rowling told her that R.A.B. is Regulus Arcturus Black. Nunez says that she only requested to know the sex of R.A.B. for grammatic reasons, but that Jo not only told her the character was male but gave her his name as well.


Source: HPFZ