I think I've seen some that have black markings. Although, Pyrex can be confused with Anchor Hocking, another glass maker.
Interesting question, though, because I've never given the color of the markings much consideration. I'm just glad they're there and I can read them. Have had a few older Pyrex measuring cups that the markings were not in the best condition.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!