Have I been blocked? My Whatsapp is showing a "Last seen" date for one of my friends, but my message didn't get delivered, as it's only showing one tick. Does this mean I've been blocked?
According to the Whatsapp FAQ:


  There are a couple of indicators that you may be blocked:
  
  
  You can no longer see a contact's last seen or online in the chat
  window.
  You do not see updates to a contact's profile photo.
  Any messages sent to a contact who has blocked you will always show
  one check mark (message sent), and never show a second check mark
  (message delivered).
  Any calls you attempt to place will not go through.
  
  
  If you see all of the indicators above for a contact, this could mean
  that the user is blocking you. However, there are other possibilities.
  We have made this intentionally ambiguous in order to protect your
  privacy when you block someone. Thus, we cannot tell you if you are
  being blocked by someone else.


I believe the fact that you can see a "last seen" status is an indicator that you have not been blocked.

Update: Per my own testing with a friend I have verified that once you are blocked, the "last seen" text completely disappears. It seems the person you are contacting did not block you and has simply stopped using whatsapp for the time being, since you can see they haven't been on since the 11th but your message hasn't reached them.