Not Medical Advice:
If at all possible, a small blister of less than 1" (even a blood blister) should not be popped, it should be allowed to heal on its own.
The fluid surrounding the area is there to create a cushion; this protects the damaged area so that it can repair itself without being exposed to infections or other things that may cause problems. By popping it, you expose the area to any bacteria on the surrounding skin, as well as in the air.
If you need to cover it to protect it from friction, use a loose bandage secured with tape that does not touch the blister.
If it's on the bottom of your foot, use a doughnut shaped moleskin pad, and if you can avoid putting shoes on, that can help allow the area to heal.