This is at least a hybrid as you say.
Mallards have the dominate Gean, which means even a hybrid that is say 75%+ black (mother black, father 50/50 hybrid) the outcome is a hybrid leaning towards a mallard more then a black.
If I were to guess I'd say its Hybrid with significant amount of Black DNA. Which is showing through even tho the Mallard's geans are dominant.
I wouldn't think this is just a poor color Mallard as this late in the year even first year birds are fully colored.
I am no duck biologist and I'm very likely dead wrong. But It's fun to pretend.