I would lean towards (b).
Personally (speaking as a brown skinned 'mixed race' Black man) I try not to look for any particular 'type' of Black woman when it comes to personal relationships. Other than that they got to be fly (to me) and irie and intelligent and know what time it is (more or less anyway- I ain't saying I know what time it is down to the second). I've dated women lighter-skinned than me (not white-never dated a white girl although I have had sexual experiences with a couple in the past), brown and black-skinned women. All of African descent except one of Indian descent. But it is true that the longest relationship I've had was with a 'mixed race' woman. Is that because of some unconscious implanted preference in my mind? Because we could relate to certain things we have in common (experiences of navigating between Black and white worlds- which doesn't apply so much to 'mixed race' people who never had significant contact with the white side of their family and never lived amongst white people)? Because of just coincidence? I couldn't really say. I must also admit that, although I said at the beginning that I don't look for any 'type' of Black woman, if I'm honest I'd have to say that all other things being equal, I find dark-skinned women slightly more attractive than lighter. Is that the result of some unconscious overcompensation for being 'mixed race', or is it because the first objects of my preteen amorous desires were mostly dark-skinned women (being that that was who I was surrounded by), or is it that black skin is simply the most beautiful skin on some 'objective' level (if beauty could possibly be looked at 'objectively'), or is it just a personal quirk with no particular meaning? I couldn't say for sure.
I realize your question was probably intended in a more generalized sense, but I am not really familiar with the particular communities you are talking about, so I thought I'd just talk a bit about how these things might work at an individual level. I din't have much time on the net right now or I might try to think more about this and go into more depth.
PS to Ayinde re the word "mix": yes I was thinking more about mix in the sense of generally having social relationships, friendship, comradeship, etc., rather than in the specific sense of mixing genes.