I think you confused the buttons a bit. It seems you wrote a reply, but instead of clicking on the "Send" button, you again filled out the username and clicked on the "OK" button (I've just change the name of this button to "OK").
You should only use the "OK" button if you want to send a quick message to a member whose username or userid you know; if you reply to a PM message, just click on the "Send" button to send it.
Admin
5. Would I get a message saying I was blocked ?
No, you will get the message if you try to send a message to the user who has blocked you.
6. Is it possible for people to send PM's, but at the same time choose not to receive them ?
You can block users on individual basis; you cannot block them all by default.
Admin