There are plenty of attractive black (that is, 100% black, not just part) men and women out there. My hypothesis for the origin of the "black girls are ugly, the cute ones are part white" stereotype:
1. Slaves come to America
2. Some of the slave women are attractive, some are not
3. The ones that are attractive are taken by white slave owners to be used as sex slaves
4. The ones that aren't attractive are left to procreate with black men, many of which were probably attractive
5. The attractive black girls give birth to attractive (as long as the white guy was attractive himself) mixed babies.
6. The ugly black girls give birth to ugly black babies.
7. Some white slave owners may have been ugly themselves, leading to a further poisoning of the attractive black girl lineage.
Plus, most black people around where I grew up were ghetto as fuck. If it's like this in most places, I'm not surprised that people wouldn't like black girls.
If you look at black people who have just emigrated from Africa (and thus, have never had to deal with the whole "be forced to mate with an ugly white guy or an ugly black girl" situation), they seem to be fairly attractive. Not that my subjective information is of any scientific standing, but I can't recall, off the top of my head, any African emigrants who were unattractive. The girls were cute and respectable, completely unlike the "hood rats" of American ghettos, while the men were handsome, strong, were well educated, and usually had accents that American girls went crazy for. This may have something to do with the fact that only the rich and educated would have an opportunity to emigrate, however.
Nigerian girls are especially cute.
http://nigerian-girls.com/wp-content/uploads/2009/09/08_oluchi_onweagba_09.jpg
tl;dr there are attractive people from any race, you're just choosing to look only at the unattractive ones in this case.