Is it Black Skin Disease?

Good day,
I suspect my 2 years old Pomeranian Caesar has black skin disease. The vets I took him to don't have an answer for me. One said it was a Thyroid problem (without a blood test) another said it was an allergy.
It has been a few months now. Is this Black Skin disease? His belly is also turning a dark color.
What tests should I have done? How can I help him? I ordered a rescue cream and shampoo and conditioner and read I should give him 1 mg of Melatonin. Is this the right approach?
He was neutered when he was 6 months. He was also recently diagnosed with tracheal collage grade 2. Thank you!

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