Hello
If you are talking about black paint on the back
of the painted area, I have a good many cels that
have had that done. I think it is to protect them
from sticking to paper, etc. I /don't/ think it
is done as part of the production process, but
it is often done afterward by someone along the
line. As I said, I have several cels that have
this same flip-side black paint on them and I 
/guarantee/ no one would have bothered to fake
some of them. ^_^
I really wouldn't worry about it. It's more 
common than you think!
Ryo