I tested this out a few times with a friend earlier. What you do is you go to someone who is offline who you think blocked you. Send them a message saying whatever you want. Then wait about 10 seconds and if a message pops up saying:
This message was not sent because Offline Instant Messaging is temporarily unavailable. Please try again later.
then this means they've blocked you. I have tried it a few times so I'm pretty sure this works (i also tried it without blocking so the excuse isn't true).
If this doesn't pop up then they havn't blocked you. I have a few people to apologise to for not believing they had computer problems now and 1 or 2 people to shout at for pretending to have computer problems lol.
Thanks for reading and if you don't believe me, try it (just don't let microsoft find out
).