I painted my computer black thinking it would run faster, but now it doesn't work.
That sounds a little off. If you had to explain the joke, what would be your explanation?
I'd explain that it must be the dumbest joke I've ever heard and it's so black, you can't help but laugh. But maybe it's not funny to you, because you don't like black people."
Even though it may seem positive to think that blacks can run fast, it is still a harmful stereotype towards them that only reduces them to one trait. They are more than that, and it would be a great idea to look up black achievements in all walks of life and areas.
I'm not sure why you're so angry about this joke. I'm not racist.
You may not be racist, but that joke makes you seem racist. Do you want people to think that about you? If not, you shouldn't make jokes like this anymore.