Is this racist?

The other day I said to my friend (who is black) that all black women are beautiful and are strong. She told me that what I said was racist. Her argument is that it’s racist because I’m saying something based off of someone skin colour even though it’s positive. She didn’t take offense to it since it’s not a negative thing to say but it started up a discussion, what really is racism. Is it all things said about a race or multiple races or only the negative things said?