Some otherwise pretty corundum gems have black in the stone. Should black greatly devalue the gem ?
It depends on the gemstone origin as well. Overall yes a black spot does effect the price by at least 50%.
And if it has more than one and combined with visible inclusions and cracks.
You can expect price to drop 70%
If the inclusions are large and interesting, collectors may take interest, and pay for the novelty of rare/unusual inclusions. Otherwise the inclusions will greatly devalue the stone.