You're lucky to have him, then! I, too, hope that he's not aggressive. I'm in a similar situation, because 3 of my newest chicks are looking a lot like roosters, and we might possibly end up keeping the most even-tempered of the bunch (although they are all untamable and wild at the moment). If...
Yes, you definitely have a Black Sumatra on your hands. How old is he/she? That information would help a little. This is our 14 week old Black Sumatra pullet for comparison and reference.
-Alexandra33