Why not? It is meant to make the engine easy to start in very cold weather but a block heater will heat the coolant inside the block which by conduction heats everything it is in contact with. Coolant is prevented from freezing though that may not be the primary intent of the block heater. Coolant far enough away from the block (like out in the radiator) could still potentially freeze if not a high enough % antifreeze of course. I always liked the circulation type heaters that pulse warm coolant through the radiator and the block and the entire circuit. A reason to prefer circulation type over the block heaters.
