Thank you for your query.
on the head one can get black eyes as described by you. They usually occur due to the gravitional flow of blood oozing in the scalp
layers. As there are no signs to get worried about you should wait and the black eyes will become normal in due course of time. Depending on the injury it may take up to a week for the eyelids to return to normal colour.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.