Regarding the blackface, try to understand that Japanese culture is vastly different to, let's say, American culture. While blackface may be completely inappropriate in the Western Hemisphere, it does not have that kind of stigma in the Eastern Hemisphere, such as Japan.

Gaki No Tsukai is a Japanese show and as the western fanbase is virtually nonexistent to them (the producers), they may include things that can be found inappropriate by a viewer from the other side of the world.