The site mentioned by you is preferred site for mongolian spots, which is absolutely benign condition.
Other possibility is of post inflammatory hyperpigmentation
, if you could recall some trauma or infection of skin in past.
It may be melanocytic nevi
None of these conditions seems to be problematic.
Whenever you feel free, take opinion of a dermatologist
so that you get proper name of the lesions.
Dr. Ajit Singh Kulhari
MD - Skin & VD