It appears to me that its the Hard fusing flux- perhaps it is tainted - I would make a sample using the same hard fusing flux on another color - repeat the same firing - and see if it performs the same way - Interesting that I do not see any bubble in the yellow -  

I also checked with the Fred Ball book and he mentions this: Blisters - Enamel may blister if impure copper was used. If blisters are glassy, they may have been caused by an impure adhesive, or one brushed on too thickly, resulting in air bubbles. Sometimes there is dust in the adhesive or on the brush. You can try to break the blisters with a sharp tool and mend the enamel.