I have heard more vicious racial comments by blacks to other blacks than I have ever heard from other white people toward blacks.
I saw that happen, and sometimes the comments were vicious, but frankly, I don't consider it racism. By definiton from dictionary.com, racism is "a belief or doctrine that inherent differences among the various human races determine cultural or individual achievement, usually involving the idea that one's own race is superior and has the right to rule others."
Blacks deciding that some blacks were less attractive than others doesn't rise to the level of racism. Hearing it in context struck me about like white people saying that they preferred blondes over redheads.
Now, don't misunderstand me - I met plenty of black racists. It's just that the particular issue of dark-skinned vs. light-skinned blacks doesn't really qualify for me.