I have the exact problem in my most recent shipment of superworms. I have never seen this before and I'm not sure if it indicates that the worm has parasites or is dying or if is completely normal.
They will begin to eat weak or dying ones, that's usually what the dark spots are from. Not a problem to be concerned about, just be sure your worms have bran or oatmeal to eat and a few large slices of potato or carrot. A small handful of greens every few days is good as well.