Thread:Support/About StatusNet:Blacklist-575fd0950270557a24cec3ddd7b388de575b84b9-Cannot subscribe to nickname "/ca/reply

Have a look at the translator documentation in the translator interface. It completely describes the use and the parameter:

'''Client exception thrown trying to subscribe to a person with a blocked nickname. %s is the blocked nickname.'''

%s in StatusNet is what $1 is in MediaWiki.