I am not sure whether this is a case of the instructions being wrong, or something is broken, or that it doesn't work on meta's implementation of a global blacklist.
Instructions at [[mw:Extension:SpamBlacklist#Blacklist syntax]] state that a caret can be used to match the start of a blacklist regex
"The '^' and '$' anchors match the beginning and end of the domain name, not the beginning and end of the URL."
Recently this methodology was used at meta in an attempt to lessen the collateral damage of a block on t.co" urls (which are routinely used to spam). [[m:User:COIBot/XWiki/t.co]]. However, it has been found that the "^t\.co\b" as tehregex has ot been effective and t.co domains have been able to be added.
It would be useful if we could work out which of the three scenarios that we have so the proper fix can be requested. Thanks.