If you use an ink sac, you get a template. Templates drive builders to lay any blocks in their inventory in spaces where the prototype had a block of any sort. Blueprints, on the other hand, lay the same sort of block in each spot that the prototype had in that spot so long as it has that sort of block in its inventory, and otherwise leaves the spot blank.
I am trying to build a railroad on a trestle, with nice lights, red power glass covers for roofing, and so on. The template just builds a big hunk of bricks or lays stuff at random, rails down at the bottom, lights here and there, etc.