NHL blackouts (at least today’s NHL blackouts) are not for getting fans into arena seats. They exist because local cable broadcasts own the exclusive rights to broadcast games in local markets.
This is entirely the fault of the leagues in question. They set up a shitty service, no surprise that many people will pirate rather than paying for a service that only partially meets their needs.
There's a common misconception that blackouts are in place to protect attendance, but that is (typically) not the case. The Chicago Blackhawks did that for a while (under ownership of Bill Wirtz who explicitly thought that showing local games would hurt attendance) but when Rocky Wirtz took over the team in 2007, he reversed that policy.
Blackouts exist to protect local media rights, which you correctly pointed out were sold to them by the league/teams.
You'll see though that many local sports networks are in financial trouble, and leagues are looking for ways to reacquire the rights to stream directly to fans.
What gets me is, if I'm in Central Canada, and I want to watch a Montreal game on Sportsnet East, it's blacked out for me! I get that it's a direct feed, but what is the point of offering me the channel if I can't use ot.
This is entirely the fault of the leagues in question. They set up a shitty service, no surprise that many people will pirate rather than paying for a service that only partially meets their needs.