People are going to start making a whole lot of blacklists that are themselves spam. It'll be confusing to a point but will likely taper off in the end.
On the other hand, its good for us who have lists off-chain as it gives an option that's easier to maintain. Currently the only on-chain blacklist is the @plentyofphish phishing blacklist that uses the mute option (the account mutes hacked accounts and unmutes them when they're clear).
What this will need is some thorough documentation for devs. I already fielded questions earlier today.
It also seems that there's some confusion of whether it'll cause services like Hivewatchers and Spaminator to lose their blacklist. This will not. It will instead build in a layer of redundancy as backup.
Being able to opt-out of blacklists I disagree with would be a dream-come-true.
The API docs for this will be pretty simple. We'll probably be able to publish them Monday or Tuesday.