Shoutbox

There is no way of telling if someone has blocked you on Windows Live Messenger.

If their status is set to "Appear Offline" and they haven't blocked you, then they will receive messages, they may not reply as there could be a reason they are set to appear offline.

You can always send a message to someone offline, whether they're blocked, appearing offline, or genuinely offline.  You'll only get an error if you try to send a message that's too long.

If contacts don't sign in when you're signed in, then you won't see any changes to display name, personal message, or picture. 

Nothing can tell you whether someone has blocked you except for asking someone else who has them as a contact, or just asking the person directly if they've blocked you.