The term Black beauty applies only to Jefferson nickels and the reason is because of improper annealing. Your quarter is a 1983 and it suffers from environmental damage and toning and it is worth only face value.
Agree, coin's appearance is due to exposure to environmental agents. I've seen a few examples over the decades; yours is one of the better ones. If you like it, keep it. The quarter could be worth a dollar or two to someone who loves to collect interesting looking damaged coins.