Didn't know that many Black people were in Frankfurt...I'm not afraid to visit anymore. Just kidding but it does make me feel more comfortable as a Black American.
Of course you can, especially in German cities with more than 200k inhabitants it's very diverse. Nobody will be unfriendly just because you're black, dude. Germans are way nicer to tourists than for example French people nowadays. ☺
Half of Frankfurt's residents are not original Germans. They are Germans with a migration background or foreigners with residence rights. As one of the European banking metropolises, Frankfurt is also a very international city.