I know a black/white mixed lady that's married to a white guy. I think you're reading too much into this. Their tastes have as much to do with class and culture as they do with color preference.
Also, it seems to me that black and white mixed people have a harder time being accepted by either group at least in this country, unless they're "pretty". White and Hispanic and Asian all mix together less obviously. I knew a kid who was white/korean, and you'd never know by looking at him. Maybe those women you're thinking of have simply had a better time being accepted by black men, or maybe there's something about those black men that just does it for them.