The use of the terms "black" and "black slave" as insults is:
A common argument to make here is that "consider how you would be discriminated against when you become the minority, you wouldn't appreciate such jokes on your social group". Although we head in the same general direction, I do not agree with this frame of mind. Discriminatory jokes are wrong, independent of the possibility of the discriminator being held to the same activities in the future.
The argument to "take this easy as these are just jokes" is ridiculous. The mere act of descriptive terms for a specific social group being used as a "joke" undermines the seriousness of the social issue. Find something else to joke about. Not something like racism and other forms of discrimination.
It is similarly bad to direct hatred towards others who fight against racism.