That definitely isn't a purebred cayuga; it has a white bib. Many crossbred ducks look like that. Here are some possibilities, just going by color:
Pekin x Appleyard
Blue Runner x Rouen
Welsh Harlequin (silver) x chocolate runner
Cayuga x ???
Black Swedish x almost anything
etc.... many more combinations make this color.
So you can't go just by color. You have to factor in the shape and weight to narrow it down more. Even then, you'll probably never figure it out. It is very unlikely to be a rare breed such as pomeranian.
If it originally came from a hatchery then it is a black swedish. If it was bred in someone's yard it probably has a parent from at least one of the following breeds: pekin, cayuga, swedish, or black/blue/chocolate runner.