There are natural gas fireplaces/heaters that don't require line power to operate, but they are slightly more expensive. Two of them helped keep my parents' pipes from freezing after a fire knocked out the power.

All an appliance really needs is enough juice from the thermocouple to operate the control system, shutting off the gas if flame is lost.

A blower is an electric motor, and that takes a lot more juice. But I have one of those self-powered fans on my wood stove and it works rather well. They are available for freestanding natural gas fireplaces as well.