It’s not racist at all. It has everything to do with the description of Snape in the book. The description helps portray Snapes Identity and why he is the way he is.
Call it what you want I suppose I don’t really care. I am not a racist.
You'd be surprised how many book characters are portrayed slightly different from a description in the book. It just seems like height, age, hair color, weight, etc are never as a big a deal for some people as skin color. It's always skin color and it's always the same crowd. That's why people here knew to expect it.
I know this actor from Gangs of London and thought he did well in the role. I've never seen him in a fantasy role so it will be interesting if he accepts. He must've showed something though for the showrunners to offer to him. It wasn't offered to him just because he was black as was implied.