A vacuum is the best insulator. Next best is "dead air". If the area in your cinder blocks is sealed, and the air doesn't exchange with inside/outside air, filling the holes with insulation isn't going to do anything. What you need is a layer of insulation between the blocks and the inside/outside wall to keep heat from conducting through the solid part of the blocks themselves. Blown Insulation does nothing but stop air from moving...without conducting heat itself. The same principle as a down jacket.
Wall insulation contains a vapor barrier and a reflective surface to keep heat on one side. That is the type of insulation you'd put on the walls next to the concrete blocks.
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