It sure looks similar to a black pastel. It is weird you would get a black pastel out of a pewter, unless the pewter's sire/dam had a mistaken identity. That could easily happen, some people might not notice, or they are traders and don't really care to know what morph it is as long as they make money.
I'm not totally sold on the black pastel and the cinny being two different morphs. I could easily see them being two different lines of the same morph. I guess if you sell it, just explain that it "should" be a cinny, but could be a black pastel because of mistaken identity in the past.
Edit
Holy cow he's a dark pewter! I would be willing to bet he's a black pewter.