Will the blower increase the heat output of our fireplaces?

Yes, aside from the fact that all our fireplaces radiate heat from their fronts, there is still heat that can be withdrawn from the parts of the fireplace  body that are closed in behind the wall. While there is an air channel that allows air flow around the firebox, allowing it to absorb the heat via conduction, this channel is somewhat restrictive by its size. Using a blower increases the airflow through this channel, thereby maximizing the heat being conducted from the back and sides of the fireplace.