Well, latex allergy is very common so I wouldn't be at all surprised to see many people having symptoms of it. I would think that a field of small red bumps is a mere rash, while large raised welts are actual hives. But, I think there is also a grey area in between rash and hive, and at a cellular level they are pretty similar events.
However, the smooth/clear grippers sound like silicone, not latex. Which leans more in the direction of being a simple heat related problem. Heat can trigger hives, just like it can trigger rash. Black fabric heats up considerably in the sun, so the tops of the legs will be more likely to have the problem. I wonder if it happens to anyone with light-colored shorts? Anyone?