Startup idea: online text editor that logs every keystroke and blockchains a hash of all logs every day. If you're accused of AI use, you can pull up the whole painstaking writing process and prove it's real.
Do you really think it would help? The kind of people who believe an "AI detector" works will just ignore your complicated attempts to prove otherwise; it's the word of your complex system (which requires manual analysis) against the word of the "AI detector" (a simple system in which you just have to press a button and it says "guilty" or "not guilty").
The more complicated you make your system (and adding a blockchain makes it even more complicated!), and the more it needs human judgment (someone has to review the keystrokes, to make sure it's not the writer manually retyping the output of a LLM), the less it will be believed.