Am a black african man. I have always suffered from painful spots around my thighs and bottoms . As a result they leave black marks.
My questions are: what triggers them
Why are they localised there
Now that they are spreading in my upper back , how can stop them from expanding
Please, I have no intention of being rude, but I have seen painful black spots on black skin before, in the areas you describe, and they were ingrowing hairs.
Of course, any kind of skin can get this problem but it does seem to be more severe in black skinned people because the hair is often thicker.
If the follicle becomes infected then it can be painful.
I suggest you see your GP - there are a number of conditions it could be.