a friend of mine was getting black spots peeking from the clothes she was trying to create & removing shoulder & waist scaler fixed it, form looking at your pics It does look like light mechanics issue & the way butt cheeks are skewing you might have a butt scaler too, remove scaler if you have & try a laying on the floor pose, if the black spot disappears then you will know its just a butt shadow being casted on standing poses.