My Lhasa has a flea allergy or hypersensitivity, and when summer finally hit this year she was attacked by a ton of fleas. Her skin got hot and red all over within 24 hours, and after that a lot of dry skin with itching followed after the fleas were gone, and during that time she went from nearly all pink, to being spotted like a dalmation the next day, and the day after that it became a bunch of big blotches and she was probably 60% black/grey and 40% pink. It looked very natural.
I read of changes with severe long-term allergies or certain infections, but they all included a loss of hair, scaly texture, and possible flaking of the skin. She had a little dryness in areas she was bitten, however it didn't go along with the black/grey spots, it was in random areas and covered both pink and black. I thought it might be dry skin causing it, but I didn't notice any change in texture of her skin between the black and pink, only around spots where she was bitten and focusing her chewing/itching on.
It's been almost 2 weeks now, she is flea-free and nearly back to normal. Residual itchiness, but I think it's a bit of a habit at this point. Right now, she is back to the dalmation-like spots of black/grey, on her upper chest and neck, and lower belly. Some of it has gone away, some of it hasn't.
Since it came and somewhat went with her allergic reaction, I was quite concerned, however couldn't find anything similar online about what it really meant. I know this is an old thread, however this is one of the very few posts I'd found with something similar - mentioning the almost overnight transformation, and not a gradual pigment change as she aged. I may not get any answers from this, but I wanted to add my experience, hopefully it might help someone else. I still have no clue why her skin changed... and then partially changed back.