def blacklist_token():
    """
    Blacklists an existing JWT by registering its jti claim in the blacklist.

    .. example::
       $ curl http://localhost:5000/blacklist_token -X POST \
         -d '{"token":"<your_token>"}'
    """
    req = flask.request.get_json(force=True)
    data = guard.extract_jwt_token(req['token'])
    blacklist.add(data['jti'])
    return flask.jsonify(message='token blacklisted ({})'.format(req['token']))