Yes, rap music promotes black supremacy ideals, because it supports a lot of things, but none of those things are living peacefully and respectfully in society. Rap music promotes the message that black people have been wronged by white people. The music even discusses or advocates for seeking revenge. The music is quite polarizing, and full of overt and covert black supremacy.
You can't just look at rap as 'black music' and you can't just look at rap as talking about one issue - it is music that can be made and enjoyed by all, even if its origins lay in black culture, and it can speak of many important issues in many communities.
No, rap music doesn't promote black supremacy ideals. Not unless black supremacy ideals include settling arguments with guns, treating women like garbage, and ranting about the police. This music isn't really meant to be uplifting or inspiring in a way that would promote such ideals. It's just music from kids talking about their experiences of being black in America.
If the question was does rap music promote misogyny, I would definitely agree. I don't think that in general it promotes black supremacy though. I think that sometimes songs express their anger of being oppressed and anger about inequality and that it completely understandable. A desire to be equal does not make it a desire to be supreme.
I do not believe that rap music promotes black supremacy ideals. If anything it points out the downfalls of growing up in impoverished American communities. I've listened to wrap for a good two decades and never come to the conclusion that rap music supports black supremacy ideals. It may talk a lot about cash, and hoes, and cars, and dro, though.