Since people seem incapable of doing a simple search on Blackface, its historical context and why it might be offensive to someone here ya go. You may not share the same sentiment and honestly that really doesn't matter but here ya go so you can't say you weren't informed. As for comedians doing it, I hate it personally when anyone paints their face. It's not necessary for me to paint myself peach to imitate Rikki Lake and to think you do is a problem, but it's not my problem.
Blackface on Campus
Blackface from Jim Crow to Hip Hop
History of Blackface
Racial Co(option)--Visiualizing Whiteness in a Suburban Space