The difference is both Helen of Troy and Snape were never real people.
Other than being beautiful Homer never describes Helen or Troy in any physical details. People are familiar with the Hollywood interpretation of the character as a European caucasian with blonde hair and blue eyes. There is nothing historical about that depiction though. If anything she should be⦠IDK⦠Greek?
As for Snape I guess he is described in the book, but the original creator is very involved in the casting. If she wants to change it up for a show then that is her prerogative. Iām not an HP scholar but I donāt think there is any story relevance to whether he is black or white.