When you blow on the exposed filament, you actually cool it off because the air current carries away a fair amount of heat energy. As the temperature of the filament decreases, its electrical resistance decreases as well. This is because the atoms making up the filament vibrate less at lower temperatures, making collisions between the atoms and the electrons moving through the filament less likely. With fewer collisions, the electrons move more freely through the filament—in other words, they encounter less resistance.
Lowering the resistance of the exposed filament lowers the resistance of the complete circuit, allowing the flow of current in the circuit to increase. Since the flashlight bulb is part of the complete circuit, current through it also increases, making it glow more brightly.
When you screw in the flashlight bulb, its tiny filament heats and glows almost instantaneously, but it takes the large exposed filament a second or two to reach maximum temperature. For the short amount of time that the large exposed filament is relatively cool and has low resistance, the flashlight bulb glows very brightly; but once the exposed filament heats up and its resistance increases, the current in the complete circuit is reduced and the flashlight bulb dims.
When you turn on an incandescent lamp, the filament starts out at room temperature. While the filament is relatively cold it has a low resistance; it draws a large pulse of electric current at first, then settles down to a lower constant current. The initial burst of current can be ten times greater than the constant current. That's why incandescent light bulbs tend to burn out when they're first turned on: The initial large rush of current causes stress in the filament.