Sorry for the delay
SB = super black. Black coppers with distinctive black fins are rare any where, not only in the USA. The first blackish copper I saw was from a copper x orchid cross. Their color were more black than copper. But the first dark copper - black copper I noticed appeared after the black dragon. So I'm guessing the black copper I have in mind or what I think you mean has something to do with black dragons - metallic dark copper body with blacker fins.
Crossing to a white, cellophane or any light color will not be "neutral" since each of those colors carry different genetic codes. They will ruin your quest in creating black finned bettas. You best bet is to keep breeding black copper, black dragon, and super black.
I've noticed that dragons work differently to regular colors. I'm still trying to figure out how to manipulate the combos they throw. Problem is, I don't know their exact genetic codes. So I'm mainly guessing at the moment.