Is saying that someone is black racist?

Yes

if you're just saying someone is black doesn't mean that they are racist they are just little bit incorrect because if you see an African American and you say hey there's a black person then that's wrong because im not black im brown but no that doesn't make you a racist

I didn't want to edit what the previous answers were because I wanted other people to see them. I say no, as long as it's used descriptively and not in a derogatory way. However, many people do feel that calling a person black is offensive and may react to being called that. So, if you don't know the person well, it might be best to either ask what they want to be described as or call them African American.