Watch your video again. He did not say... "If you vote for Trump, you ain't black"
He said, "If you have a problem figuring out whether you're for me or Trump, then you ain't black."
The sentence isn't racist because he thinks only black people are voting for him. Which, perhaps, he may actually think that.
The sentence is racist because he thinks black people had, at that point, made up their mind.
Stupid for him to say? Yes. Racist enough to not vote for him? No. Not even if you were black.