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Oh...duh... Can you put in a page on /user with a form input? So there is a link that you can put in and users that haven't used it before don't feel like a dumbass. XD

It's really designed to be a rest api and not something that is used interactively but it is possible to just put a username there and do a quick check.

Explaining something like
"Put your username instead of XXXXX below:
http://blacklist.usesteem.com/user/XXXXX" may be more helpful