That is what the dark web is for, makes no sense to blackmail someone on the block chain as it is publicly visible and it would already be out there forever.
Encrypting image, putting it on the blockchain and sharing the key with the victim looks like a good blackmail mechanism to me.
Anything that is not possible to censor or redact has that feature, even our comments now, everyone can put out threats, blackmail if they want to in plain text.
It would be easier to just put the SHA256 hash of the file as proof of existence, it is already possible to write any data to the BlockChain or any other storage system online.
