User blog comment:Aryeh.wolfblade/The missing legions aka fergotten ones./@comment-3416287-20120119021813/@comment-2253300-20120221155239

Actually there is some hinting that the Blood Ravens may be loyalist remnants of the Thousand Sons. Mind you i forget what the quote was, or its source, but apparently a member of the Thousand Sons had a vision about the future of the Legion, and he mentions "Ravens of Blood". However this is not confirmed, and mainly speculation on the part of the fans.