… and they try to send you something, does the e-mail bounce back to them with a message that they’re blocked, or does the e-mail just get deposited in the Great Void?
Depends how you block it. If you block it in your client, it will probably just be deleted upon reciept. If you block it at your ISP, or using a service like Hotmail, the sender might recieve a bounce message.
Here’s a suggestion on how find out what happens. Block one of your e-mail accounts. (If you don’t have another, just set up a hotmail account.) Then try and see what happens.