That sort of makes me wonder if the best form of censoring would be solid black shape, THEN passing it through some diffusion image generation step to infill the black square. It will be obvious that it's fake, but it'll make determining the "edges" of the censored area a lot harder. (Might also be a bit less distracting than a big black shape, for your actual non-advisarial viewers!)
I think the edges would still be evident, and this would just waste time and energy. I think a black square is just fine, so long as you can leak some information on the length of the secret. I would make it larger than it needs to be.
