If it was an improperly annealed planchet, then both sides would look the same, darkened. Also, the biggest tell that a coin is improperly annealed instead of just toned is that the edge of the coin will be shiny like an UNC nickel. There should be mint luster still present on the coin, which yours lacks. This all adds up to me thinking your coin just being a circulated coin with possible environmental damage. Put the nickel between two UNC-like nickels and take a photo of the edge of all three coins. Here's mine for comparison, sorry for the poor photography, these are the seller's photos.
Sorry, but the side of the coin should not be dark as well. It should be virtually indistinguishable between the other coins. Also, the obverse and reverse should've been uniformly dark. The sheet are annealed and then the planchets are cut from them, meaning the edges were not exposed to the heat/oxygen. This is why edges of the coins should look like a regular nickel. Sorry, your coin is not an improperly annealed nickel.
