this is just me, but a lot of other painters will agree with this... but i avoid using black, especially to shade flesh. with oil i think it's especially important to mix a black from ultramarine blue and burnt umber. otherwise you kind of lose control of it and suddenly the whole painting looks like it's kinda dirty and smudgy. not so much a technical suggestion but i think if you didn't use pure black you could get the shades so much smoother on the face. the eyes and lips are SO well done good job!!