Black patches or spots can be the result of a few different causes. Small bruising can cause it; mild infection, usually the result of high humidity, or scratches from other cage mates, is another possibility. Mites, can cause black patches as the cut off the blood flow to areas of the skin. To rule these out, ask yourself these questions:
1. Are prone to frantic behavior and running into things when startled(very common with this species).
2.Would you describe the cage as wet or damp, do your lizards feel clammy or dry when you first take them out?
3. Are they often on top of each other? Have their nails been trimmed? Could they be mating?(biting to the nape may be a cause).
4. Looking closely can you see small specs of white or black or red, usually around the eyes and mouth. Animals with mites usually have very irritated looking eyes.
Hopefully it's nothing serious, good luck!