That's what all browsers do and i'm sure mobile safari will do it too once the feature arrives.
As long as the default is to globally block and that enabling for a single site is as cumbersome as possible, that's okay.
Personally I’d like to run a server that could send notifications to me and to other people who would like to join this party. It is a valid use case and making it as cumbersome as possible feels wrong (depending on what it means of course).
Though I agree that it would be good to not turn it on by ok-clicking like our parents always do. “aA -> turn on” would be enough, popups definitely banned.