you can find usefull information here - some alternatives for messengerdiscovery.
http://shoutbox.menthix.net/showthread.php?tid=42...d=433314#pid433314
the plugin "openconvo" that i give a link to in the thread above can theoretically detect blocking just like messengerdiscovery did.
(when the user opens a conversation window with you, you will get two notifications at once)
EDIT:Block checkers don't work
Edit: also try out stuffplug,
http://www.stuffplug.com/ it has a variety of cool features you might and probably will find helpful
go here for more plugins:
http://www.msgplus.net/pluginsdb/down/? 
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