I'm not entirely sure. On the one hand you could just say that you only have "detail" blocks, and a detail block where all the AC energy is zero is a "smooth" block.
But there are some things about "smooth" that I think deserve a more global consideration in the coder as a special mode. I will try to ramble about them a bit :
1. Smooth blocks need more DC resolution. With no detail in the block the eye is much more sensitive to the color being wrong. In fact DC quantizer should go up with AC energy in general. In typical coders the DC's are all sent separately from the AC, before them, so that order should be changed. Either signal block type before sending the DC's, or send some information about the total AC energy.
1a. They particularly need more DC resolution when they have a similar DC to their neighbors. This can be done by sending the delta-DC with a non-uniform quantizer, which is a hack I wrote about previously.
2. Even with lapped transforms or standard deblocking filters, a zero-AC block is lumpy. They use filter shapes that make the block flat in the middle and sloped at the edges, which gives them a C1-smooth stair-step look.
3. Connected regions of "smooth" should be handled specially. They should have restored-with-quantization range maximum-likelihood gradient restoration.
Say you have a DC quantizer of 8, and you have a row of smooth blocks with DC's of [ 0 0 0 8 8 8 ] . Current methods will give you a run of all 0 pixels, then a smooth ramp up to 8, and then a run of all 8's. Obviously what should really be there is a long smooth gradient.
4. A "smooth" next to an "edge" block can imply that the smooth gradient region goes up to the edge found within that edge block and no further.
5. If you get to such low bit rates that you take a detail block and send zero AC's just because you can't afford them, you don't want that block to be considered "smooth" and put into the big-smooth-gradient pathway. Though really this needs to be something your RDO fights hard against (don't let detail blocks go to zero AC's even if it seems like the right RD decision because it actually isn't under a better D metric)