I've been to three different indoor ranges in the OKC area, and all three allow black powder.
Although I got told at H&H that only one of the bays were for BP, by one of the employees. He also told me I couldn't load anywhere but at the firing line. I tried to explain to him I wasn't loading, just charging, and I wouldn't cap until I was at the firing line. He couldn't grasp the concept, so I didn't argue with him. After finishing up, I found a manager, told him what had happened, and he said he'd have an instructional talk with the employee. He also told me all the bays were open to BP.