I've seen these ducks together, but I don't think they are actually related. The dark one appears to be a young Black or Black hybrid. It's smaller than an adult, and last time I saw it, there was some downy feathering on the back. I don't know if the facial pattern is a result of young age or hybridization with something else. (I don't often see young Black Ducks.)
The leg and eye colour of the white ones makes me suspect domestic duck genes rather than albinism, but I haven't seen them associate with any adult, so I really have no idea.
BB
