Is it okay to sing Summertime if you are not black?

If you want to sing it, thee is no reason not to sing it.

"Summertime" is from the Opera "Porgy and Bess," which was written to be performed by a black cast (although it was written by a white composer). But that doesn't mean a white person can't also sing the song. Blacks can sing "Scotland the Brave," Americans can sing "Wem Gott will rechte Gunst erweisen," and Germans can sing "La donna e mobile."