Hi David,
I am a former lovie owner and I was wondering have the feathers themselves darkened, or are you able to part the feathers and see the dark area on the skin?
If its just the feathers maybe he leaned on somethin or maybe it is a color that is just now coming in. If its on the skin then you may wanna have a vet look at it or try to contact an avian vet and just ask.
pjay