Blackening your skin to imitate a black character is not racist. It's just doing everything you can to make your costume an accurate portrayal of the character
"Black Face" is a very specific western concept involving blackening your face in to create a racist caricature of black people. Specifically it rose to prominence in the early-mid 1900s in America, where it was used to mock African Americans during the time just after slavery was abolished.
Context and intent are extremely important and it's sad that our society has become so reactionary as to completely ignore all context and intent as people spasm about like epileptic children at the sight of something that MIGHT MAYBE POSSIBLY be bigoted, without bothering to take stock of the situation.