Yes, Alan. But my own darkcloth is a little heavier version of it. Goretex is Goretex. It's not a stretch material, or cordura, or ordinary ripstop nylon, but a patented line of tri-layer high-quality material that rain parkas etc are made from. I can't vouch for knockoff brands. But since this is a minor application, and not a matter of life and death like a high altitude tent, there might now be certain similar fabrics which would also work. You want to search under Black Goretex and then read the fine print. It's relatively expensive per yard. I use heavy-duty velcro at the corners.
If someone wants something dirt cheap but very light (too light in my opinion), breathable, and waterproof for a awhile (a few hours at a time), there is a kind of Tyvek truck wrap black on one side and white on the other. I used to grab big scraps of it headed to the dumpster after the lumber truck loads were upwrapped, but more for applications like groundcloths under a tent rather than darkcloths per se, though this kind of material would work far better than anything cotton, because it does not produce lint and does not absorb water easily, and can easily be rinsed off if dirty.