Is it true when bottoms get fcked so much their round hole resembles a slit?

Nope definitely not, just porn logic/urban legend in yaoi.

Bc it's a muscle that can be trained, someone bottoming frequently but safely will have no changes, it'll just contract back again (like how ballerinas can do the splits but they don't walk funny all the time). Strains/tears from extreme stuff can damage muscle and that could lead to swelling while it heals, but no slit shape at any time ╮(￣▽￣)╭

How about intentionally darkening the hole. Is that something that can be done

Ummm I honestly have no clue. Based on my best guess tho, probably?

For context, many ppl have darker skin there and are self conscious, so there's a fairly large market for anal bleaching products, which I don't know if you're aware of...

As for darkening on purpose I doubt there's dedicated products, but where there's a will there's a way lol. Also if you're talking about immediate change, rather than long term, then sex/fingering/etc will definitely make it red for a while, maybe even a few hours/days.

Hope that was helpful hahaha

Zammm yea that was helpful. For someone not well versed in the department, these ero books can get misleading.