Well, the Black Court are essentially a breed of crazy powerful zombie, and are definitely undead, so its hard to say. To my knowledge Reds are considered irrevocably evil once they turn largely because they loose their soul completely, and thus are more of a monster with the body and memories of the original. Whites by contrast retain a soul, and are born as a half-human, half-demon, and basically a just a type of changling that breeds true.
GS Spoiler:
Uriel refers to them as "The scions of mortals and immortals. Halflings, half-bloods, half-born." Which basically sounds like a Changeling to me, so Im not sure you can add something else to the mix.
The thing about a Changeling is that (as per WOJ) eventually they all make a Choice, either consciously or unconsciously, and become either mortal or not, so sitting on the fence too long may not be possible. That being said, Drakula was a changeling (or at least the son of one) who ran off to become a Black Court as "teenage rebellion" so mixing may be possible for Blacks at least. Who knows on reds, but its kinda moot now