The air you blow across hot food is at a lower temperature than the hot food itself. At first glance, it may seem that blowing this cooler air across the hot food is what cools the food. Not exactly. When you see steam rising from your food, this is the evaporation of water molecules into a gas that then quickly cool to form tiny water droplets in the air. The steam you see is the collection of tiny water droplets. As these molecules escape, your food cools down. However, many of this molecules 'fall' back into or onto the hot food. The food is slowly cooling as not all molecules return to the food, but the process can take awhile.
To cool your hot food more quickly, you blow across the food to move these molecules (steam) out of the way such that they don't return to the food. If the molecules don't return to the food, energy is not added back into the food, thus the food cools down more quickly.
Common sense and daily experiences tell us that blowing on food cools it down more quickly. Now you know the WHY behind it. :-)