AS per the description you have given, and assuming that the size and location of the mark has not changed after its appearance, the bruise
like appearance is mostly due to a type of birthmark
called MONGOLIAN SPOTS also called Congenital Dermal Melanocytosis. It usually disappears in few years but it may persist in some cases till adolescence. This is not associated with any complication and no treatment is required for it.
But you should monitor it and see a dermatologist
in case of following changes
Hope this helps you. Take care