What was your original firing cycle?
It appears that your black glass has an iridescent coating. Is this correct? If so, I would try fixing it by sprinkling a layer of clear powder over the whole piece, and refiring it. The clear powder will flow over the irid coating, making it less satiny and more sparkly.
Relevant technical resources from Bullseye: Quick Tip: Glimmering Irids
:http://www.bullseyeglass.com/methods-id ... irids.html
The effect is not devitrification. However, the same method that we recommend for fixing devitrification may help in this situation: Quick Tip: Using Bullseye Clear Powder to Fix Surface Flaws
:http://www.bullseyeglass.com/methods-id ... flaws.html
I hope this helps!