It is very rare to get an ALL black ferret. You will always have a little bit of light mask on a ferret. Although baby ferrets can be all black but change with lighter marking within a few months of age. There are ferrets out there that have a lot of black but not ALL black unless they are mixed with a polecat, thus being a hybrid.
Could it be a polecat, mink or something of the like? They are often found to be all black and look a LOT like a ferret.