There are LOTS of different Red Tail localities; there could be a chance that you maybe have a cross of one. Without knowing or being able to prove out the genetics unfortunately it'll be hard to tell. Your little one getting SUPER dark is something I don't see, but every boa is different. Usually the ones that get that dark are something more than just your average BCI. The tail pattern looks more BCC-ish to me though as well, but the coloring's not vibrant enough imo. Did the seller have any information on this guy/gal? Or any info on the parents? That would be the best (and easiest) way to find out if you have more than "just" a BCI.
Either way as long as you're happy with him/her, that's what counts.
EDIT: I keep looking at the last few pictures you posted and I dunno, it looks like more than a BCI to me. But I don't think it's BCC either. If Evenstar doesn't get back on this thread, I might shoot her a PM and see if what she thinks, if you're curious.