War nickel blackened on one side

I recently came across 3 silver war nickels when I was sorting a  bulk lot.  One coin has better detail than the one I already have in my collection.  The heads side of the coin is beautiful and clear but the building side of the nickel, although it has nice detail, it is really blackened.  

Anyone know what would cause one side of the coin to discolour?  Whether it affects the value of the coin?  And whether I'd be better to keep the old one in my collection or replace it with the blackened one.

I think it blackens because of it's composition of Silver-manganese. Most of my war nickels are pretty ugly looking actually. VF+ to UNC war nickels are obviously far more expensive than the "blackened" ones.

But it's only blackened on one side of the coin.

I've heard it happens to coins that have been stored in trays, one side is exposed.....

I think that's correct. Somewhere this side of coin was exposed to open air.

Oh ok, does it affect the value of the coin at all?  I'd grade it at a solid VF+ otherwise.

I think it does unfortunately. This is a very common coin, so the grade does matter.

I know rubber bands will leave black marks on silver. I wonder if leaving a silver coin on a piece of rubber would have the same effect.