Update 7/13/21: I took some additional pictures of the plug, as well as the pipes the plug is used on, and added a description.
I recently turned on my irrigation system after it having been winterized last year. I did this via a blue-handled valve located outside in an access box sunken into the ground with a large, green pop-off cover.
Much to my surprise, when I opened the valve, I was shot in the chest with a jet of water. After shutting the valve back off, I saw that there was an open pipe through which the water had shot. It had a threaded end and attached to it was this black rubber thing.
The plug itself is made of two parts. The first part is made of a soft, pliable rubber with a loop at each end. The larger loop is designed to stretch around a pipe and keep the whole thing from dropping to the ground when it's unscrewed. The smaller loop effectively functions like an o-ring. The second part is a hard plastic thumb screw. This screws into a pipe in order to press the o-ring flush with the opening. The top side is chewed up from when I tried to tighten it using a pair of pliers.
I tried to screw it back on, but when I opened the valve again, it must not have been making a good seal with the pipe because small jets of pressurized water were shooting out from under it. I spent a while mucking around with it, tightening it to varying degrees, but it always leaks.
What is this thing called? I've had no luck googling for variations of "seal", "cap", "bleed screw", etc...