Well, it was normal in the 60's. This isn't the first time that an African-American woman has played the Cat Woman role... hello, Eartha Kitt...
Yes, the "original" Selena Kyle that debuted in Batman #1 [1940] is caucasian, but you're being really nitpicky here.
Besides all of that, the premise of this movie is apparently that Selena is dead and that Patience is taking over the role of Catwoman, so technically it is a new character.