There are basically two ways to design a 2D graphics system:
Provide lots of hardware support in the form of tiles, hardware scrolling and sprites, to put together each frame on the fly from a small quantity of data, so you can have lots happening on the screen in every frame even if the CPU is slow. The Commodore 64 and NES are well-known examples of this strategy.
Provide a simple frame buffer that can store a relatively large quantity of data, and rely on the CPU (plus any support chips) to be able to move that data around fast enough. The Apple II and Amstrad 464 are well-known examples of this strategy.
The second strategy produces an elegant and flexible design, but has the disadvantage that if you want lots happening on the screen in every frame, you can get severe slowdown. For example, a 4 MHz Z80 can blit about 200K/second, which sounds like a lot but at 60 fps, it would be only about 3K per frame, and you really want something like 20K of frame buffer for good graphics without tiles or sprites.
It is said that the Z8410 DMA chip can blit much faster, up to 2 megabytes/second. Would it be feasible to use it as a sort of GPU, put it in a home computer and let it handle scrolling and suchlike for an order of magnitude speedup? If so, why was it never done?
It seems the answer is yes, doable! But a couple of people have pointed out it's not a full match for the Amiga's blitter, because it doesn't do bit shifting and masking, only byte transfers. This is true, but will be considerably ameliorated by using chunky pixels (as opposed to bit planes on the Amiga). Suppose we had one pixel per byte. The problem would disappear. Unfortunately that's not feasible on an 8-bit home computer, but suppose we have something like the low resolution mode on the Amstrad 464, two pixels per byte. Now we can move two pixels without bit shifting. Better yet, if we double buffer, each buffer can move two pixels without bit shifting, for a net effect of one pixel movement per frame.
Addendum: if the blit is for the purpose of horizontal scrolling, hardware scrolling - which I think is cheap to provide? - would also help.