Requested feature overview description.
The api endpoint

/user/blacklist.json

already exists and works with api login credentials

though it is undocumented

so

https://e621.net/user/blacklist.json?login=wolfgang_senizen&password_hash=********

returns a correct blacklist

but the endpoint does not support the jsonp callback param as referenced here

Why would it be useful?
Would bring the endpoint inline with other ones

Would allow it to be queried in other sites without having to pass the login details to anywhere other than e621

(atm to query it requires a proxy if attempting from javascript on a different domain)

What part(s) of the site page(s) are affected?
only the

/user/blacklist

endpoint, and the change won't break any current functionality.