Most manufacturers' blocks are reversible top and bottom. For example, a corner block can be flipped over to create a left or right corner.
Because parts of the foundation would have to conform to irregular ledge exposed in the excavation, we chose a snap-together ICF form. This allowed us to cut the outer and inner panels separately to fit the contour before we snapped the two panels together with plastic ties.
We cut the inner panels lower to create a turned-down curb around the perimeter. A 1-foot-by-1-foot rebar grid supported the heavy weight of the pool; vertical rebar couldn't be used because of the ledge below.
We installed radiant-heat tubing that was connected to a separate zone on the boiler just before the pour.
The blocks we used (rewardwalls.com) were 16 inches high and the bottom course was ripped to 8 inches high, yielding—with a full block on top—an overall height of 24 inches.
Unlike the forms for the pool slab, these came completely assembled, so I stacked both courses before shooting elevations ...
Because the outbuilding that would top the slab would be heated, we placed 2 inches of rigid insulation over compacted mixed stone with fines before pouring the concrete.
We laid 10-mil Viper VaporCheck) on top of the foam and taped the seams to control moisture migration into the building.