I suspect it is the same unknown illness among Ember tetras. It may look blue or black under different lightings. You can even spot some Embers with the symptom in an ADA official video on YouTube.
IME, some started with a tiny black dot, some started with black line along the body. Then the fish would lose its color. If it reached the gills, the fish would die within days from hypoxia. Otherwise, it could take months. They would eat, swim but they were slowly wasted away.
I've been searching the web for years without finding a good answer. I think it might be tuberculosis.