Make sure the Vertex normals aren't locked:
Choose your problem vertexes, then:
Mesh Display > Unlock normals
Then try to flip the normals again:
Go to face mode and select the dark faces then in the top menu go to Mesh Display and under the "Normals" category click reverse
AND/OR
Choose: mesh display > set to face, which usually fixes the reversed normals while also leaving the normals which are fine
If reversing the normals doesn't work, your vertexes are probably locked. Select problem vertexes and go to:
Mesh Display > Unlock normals
You could also try: Shift+RMB go to the bottom and reset normal settings
References:
https://www.reddit.com/r/Maya/comments/51ajr5/why_does_my_objectface_turn_black/
https://www.reddit.com/r/Maya/comments/ab2zfb/imported_object_has_reversed_normals_cannot/
