Aaaaaahh... Now I see what you mean by those black spots! I have this, too.

I didn't worry about it, because I had the same already in my N8, so I just think it is a common AMOLED issue (screen bleeding, or however you want to call it)...

Therefore it doesn't bother me, because it's only visible to me in rare conditions (using the phone in pitch black night, which is really seldom for me).