Hi Emily. Blocked from where/what ?

I have no idea. Nobody has ever blocked me. Don't you get a message or indication of some sort ?

I would logon to a friends IM, logon to my IM, and compare and see if the person is online on one and offline on the other :-D

Emily, you're not likely to get much help on HTG for two reasons. If
the app works in even a slightly unethical way, we won't allow the
posting of it. And secondly, we regard privacy very highly here. People
that block IM, remain invisible to their friends or use BCC to send email
expect privacy. Those exact same options are available to you, as you know.

That being said, it is highly likely that these apps exist and can be found
without too much searching (maybe save for the BCC one).