I just learned this in
PS for Photographers workshop in LA Thurs.
Create a curves adjustment layer, target the hair comd click/ctrl click on hair and darken until it is as dark as you would want any part of it to be.
Fill mask with black.
Get a paintbrush at low opacity (10-20) with a tiny brush paint across each eye lash to darken also works well on edge of eyes, lips.
Use a large soft brush to add contrast to peoples hair.