What Hopper does is
blocking everybody except who you want to send the toasts to and then setting your status offline/online a few times. blocking/unblocking the contact a few times.
All that is really not needed and overdoing it imho. And not really useable with large contact lists anyways if you want to check all contacts. Even with setting your status to appear offline and back to online it doesn't always work since this isn't a garantuee that an open session will be created.
But who said anything about detecting blocking?
This is exactly why 'open window notifiers' and all the other things, like a possible 'detect appearing offline contacts', should only be made and be used with great care... and as such I'm personally not very fond of such tools in the wild at all, even just for this sole reason only; They are used more inappropriately than that they are used and interpreted properly. See all the 'open window notifier' hypes and discussions. This has a big potential of getting on the same road.
When someone is appearing offline it does not automatically mean they are blocking you though. Please do not make such assumptions.
EDIT: corrected big mistake, thanks to Mnjul for pointing that out
/ 
