If you aren't actually trained in anything, than nothing. You might as well let him ragdoll you around willingly, because you are sure as hell not going to hurt him.
However if you actually are a martial artist, that's a bit different. Karate is a very defensive martial art, focusing on a lot of counters. The best advice I can offer you is to not try and be the agressor. Wait until he tries to attack you, or fake him and then see if he takes a swing and do some countering of your own.
However, if he's a blackbelt, he's going to be a bit more trained than that....
Good luck, anyways.