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He's definitely going to make Blocking a feature available on a higher priced subscription tier. Take it away, say people "convinced" him that some still want it, make up some BS about how maintaining the feature has a cost to Twitter and make people pay for it.
It'd probably cost a lot as he'd code it- each time a tweet is pulled up there'd be a full search to see which users have the tweet author blocked. If the returned list has the viewer's ID included then set the style for the tweet to not be visible.
Check is done by querying the list of users and running a for loop to see if any user names match the user name of blocked users. This is all handled on the client side and optimised by exposing the SQL server. Security is maintained by banning anyone who uses the word "drop" in any query (after the query goes through).