From what I think I understand from Wikipedia what a blitter is:
Remember the super nintendo? It could handle side scrolling games like Mario and the PC couldn't do that... The reason for this is that the PC didn't have a blitter.
What does a blitter do? A blitter is a microcontroller (very tiny processing unit) that would handle 2D images. So a blitter took care with moving 2D images (sprites). Mario is full of sprites.
We all know the CPU isn't really good at graphics. It's serial execution and not parallel. So if the CPU would have to take care of those 2D images it wouldn't be able to do other things in the mean time.
So... if I am right, correct me if I am wrong, than this blitter module turns your r300 graphics card into a very expensive Super Nintendo Entertainment System, otherwise know as a SNES ;)