A fireplace blower could be blowing out cold air instead of hot air due to a problem with the heater coil present inside the blower grate. The coil might have broken down, causing cold air to blow. Another possible cause could be that excess heating has caused the condenser and the evaporator to become frozen.
If the blower fan gets inoperative or switched off, the hot air stops being blown out, giving place to cold air instead. The pilot light also might have been damaged due to some internal defect, causing cold air to come out instead of hot.