Any ideas why this coin is toned black please?

2014 shield fifty pence uk coin 

found in change from shop 

please Advise do you think it’s a silver coin? I will test it later in the week but before I damage it I would like your opinions . All fifty pence coins are copra nickel except the proof ones . Never seen a copra nickel tone black ?????

Cupronickel is not oxidation proof. If you need more examples, I might take a photo of others. Dunno how previous owner had them degrade, but I would suspect humidity or steam.

Looks like a chemical reaction that has attacked the nickel in the coin 

Unfortunately, It has environmental damage.  

Left outside or exposed to something reactive. 

I have a few US nickels and dimes that look just like that.