Basically, blocks with the same colour/pattern on the front as on the top. Road already does this, but any area of road large enough to hide anything decent is somewhat suspicious anyway.
Anyway, the point is to be able to make structures blend in completely with the surrounding blocks (e.g. grass, water, sand, etc.) to make camouflaged buildings. Obviously you can still walk into them by accident :P
I'd especially like to see a block exactly the same colour as bare ground for this, as well.