It doesn't mean %100 that you are blocked, if someone removes the right for you to see them online, you also won't see them on the friend list on the website dashboard. But if they are online and they removed the right for you to see them online, you just IM them, if you get the notification that they are "offline" they are truly offline.
If you suspect you have been blocked, you can send them an inventory item, note, or whatever. If the item get's rejected immediately, means you have been blocked.