I mean, I can tell you have some black in you.
Why would you make such an assumption about my skin color?
Because there's only one human race and the color does not matter. It's a joke, but it's not for you to judge.
Making assumptions about another person's race isn't okay.  The person could be offended at the assumption you made or just think its in poor taste to do so. If someone wants to tell you their race, they'll do so.