There is an article here on how to find out if someone has blocked you, but my theory is if they have blocked you then they can't be much of a freind anyhoo
http://www.mess.be/ you will be looking for
Main/News > MSN FAQ > Contact List & Status > Can you see who blocked you on MSN Messenger? (you are here)
Can you see who blocked you on MSN Messenger?
My status is always "appear offline" in messenger when I am online.
but it dont mean I wouldnt talk if someone pm'd me