I don't think it is a pigment factor. In other words, I don't think their is any difference between clear ULS and black ULS.
Gordon states the difference is between ULS (E-glass) and ULZ (S-glass). When buying black glass, I never order ULS. I order black only in ULZ. Clear isn't available in ULZ, but "natural" is. Natural however isn't clear...but cloudy. Who wants that? No one...so for me it is either ULS clear E-glass or ULZ black S-glass.
When comparing those two, ULZ is superior. Even though it is stronger and more durable technically, I don't think the difference is worth getting excited over...probably only 1-2 fps difference in efficiency. Both are very durable; however, if one wants the "best performer," ULZ (S-glass) wins.