I always have this problem with any green or teal based boro color, whether it is a striking color or not. The teals seem to be worse about this than the regular greens. (In fact, the soda lime aqua/teal colors are prone to bubbling as well.) However, I do not have this problem when encasing, just when the color is exposed in the flame for longer periods. My flame is definitely more on the oxidizing side, and it seems like no matter what I do, it happens anyhow. I just avoid using those colors on the surface now.
For example, I was using GA Teal and it did the phase separation/devitrification thing (not sure exactly which term is correct for this phenomenon) in specific spots, mainly where there was no constant full-on contact with the flame. Like in places between bumpy dots, it would get kind of chalky and bubbly looking, and no matter what I did it wouldn't melt away. It seemd that the places where the flame couldn't get it hot enough was where the nastiness happened. I've also had this problem with NS Glacier Blue, GA Emerald and True Green, but as I said, not as bad as with teals.
Now, if this is happening with black, might you be using the GA Black/Green? I've found the GA Black/Violet to be the best black I've tried, and it comes highly recommended by others. The only problem you may have with GA Black Violet is with dichro - from what I have read, B/V has manganese or magnesium or one of the elements that start with "M", and this will react with the dichro, creating a weird outline where the dichro touches the B/V. Other than that, you should be fine. It's very smooth, and I've never had any pitting or devit or whatever.
Sarah