I am a white guy. As far as "why is it that men of different races want a black woman who looks white? Is there something wrong with black women who look black?" all I can tell you is from my own perspective.
Really dark skin isn't in and of itself a turn-off.
Big noses and big lips are, however.
So is a big ass and enormous thighs.
Afros do not look like something I want to run my hands through, bury my face in and smell.
It looks really weird when the soles of your feet and the palms of your hands don't match the rest of your skin colour.
Many young black women look totally hot from the neck down.
There's more to it than looks. There's posture, accent and a whole array of other socialized behaviors that are off-putting to people who are not "accustomed" to blacks.
There's the ever present aggravation of having to not offend you or hurt your feelings, because any little incident could possibly end up as a federal case with Al Sharpton waving a placard in your front yard. This is called "White Guilt", and is actually more of a fear and paranoia than guilt.
It is a well known fact that all black people know each other. This causes other races to be paranoid around them. This is confirmed by the fact that whenever you insult an individual black person, you end up apologizing to their entire race.
Black women with lighter skin and more "Euro" features look more familiar to white guys like myself, and make us feel comfortable. The more there is to remind us of the difference, the more we're likely to treat you as the avatar of an entire race instead of the individual you are.
And there is also the "I just don't know" factor. I look at a lot of porn on the internet. There are often black women on the porn sites I look at. I do look at them, but it's mostly curiosity. Why? Because no matter how long I look, my penis will not get hard. It is a rare instance that I see a black woman (in porn or IRL) that causes my loins to stir. That is not conscious, that is Mr Devil-in-my-pants making decisions. The black women who DO elicit penile hardening are normally the light skinned, "white" featured types, or more normally the "woman of ambiguous ethnicity" that you see in a lot of t.v. commercials.