Strange coin appearance

Found this it’s a 1776 1976 half dollarwith black finish and what appears to be a blister on reverse only any thought TIA

 

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My first thought would be heat damage. Clad coins are prone to blisters like that when they get hot.

 

Heated with a blow torch

 

or cought in a fire, i had a bunch years back that looked similar to your coin that i salvaged from a house fire.. heres the morgan from that fire that i kept..