If you are white (or not black), it’s very important that your Halloween costume not incorporate blackface. Blackface means painting your face brown or black as part of a costume of a black character.
Blackface is racist because it is part of a long tradition of white people dehumanizing black people. White people put on shows where they would paint their faces black and act out extremely racist stereotypes for their entertainment.
This was an extremely popular form of entertainment among white people, not limited to particularly horrible people. Blackface as a form of intentionally racist entertainment has become a lot less socially acceptable since the 60s, but it has not died out completely. (And many people are old enough to remember it being extremely popular, and many more people are old enough to have parents who remember that.)
On Halloween, some white people continue the blackface tradition. They paint their faces black or brown and dress up as racial stereotypes. (Eg: calling themselves thug or ghetto). That’s wrong.
But even if you’re not doing it on purpose, even if you don’t mean to dress up like a stereotype, if you paint your face as part of dressing up as someone who is black or a character who is black, you will end up dressed as a racial stereotype. You will end up participating in the same tradition of mocking black people, together with the people who are doing it on purpose.
For instance, if you admire President Obama greatly and want to dress up as him for Halloween, you would still be invoking a racist trope and hurting people if you painted your face brown as part of the costume. If you want to dress up as the President, dress as the President, not a racial stereotype.
Some other forms of face painting are different. For instance, it’s ok to paint your face white and dress up like a clown. That’s because clowns are just clowns; they’re not part of a tradition of dehumanizing others for entertainment. History and symbolism matter, and they’re about how they’re perceived as much as your intentions.
If you use blackface in your costume, regardless of what you intend to dress up as, you will inevitably actually be dressed a racist symbol, and that will hurt people.
If you need ideas about other things to dress up as, this post on costumes might be a starting place.