Are black people obligated to like Madonna? No.

And let's not forget that Madonna has talked shit about a lot of successful black artists far more talented than she ever was

She dragged MJ and Janet Jackson through the mud in the early 90s
She dissed the Supremes (even though she said she loved Motown music)
She insulted Whitney and Mariah
She harassed Tupac & even begged him to give her a baby WTF!!!

The only huge black artists she didn't disrespect was Prince (mainly because he was provocative and sexually explicit like her)