You have some options:
Preview Box Script
Keyflasher Script
And the third option was what I went for.. though is a TAD more difficult which is actually program something like xfire using directx injection.
The project is kinda sitting on my computer because I haven't had much time to do this but maybe I can give it another shot.
This is a screenshot:
It's in portuguese cause I'm Brazilian but notice on the upper left hand side that black box was actually a comercial banner from blizzard. It's a black background with blue letters those are the messages I received from my friends.
What I was doing then was installing a keyboard hook and using it to send the messages without having to actually send them in game (so the other people didn't see my chat) and it works but I only managed to make capital or lowercase letters because I'm not very familiar with keyboard hooks so far.
Notice that this is a two-part project: 1 - the program that does the directx injection. 2 - the script that reads the messages I sent (I send my messages to port 10000 and use a sniffer to catch messages in that port and send them to my contacts).
I'll try to give this project another shot this weekend please let me know if you know anything about visual basic maybe you can help me out since this is the language I'm using.
Thanks