A milk blister is a blocked nipple pore. (You might also hear it called a “nipple blister” or “bleb.”) This forms when skin grows over a milk duct opening, backing up the milk behind it. A milk blister looks like, well, a little blister. It is usually a painful white, clear, or yellow dot on your nipple or areola. If you squeeze your boob, the blister may bulge. A milk blister sometimes sticks around for a few days (or even a couple of weeks) before healing on its own when the skin peels away.
If you have a blister that’s red or brown, it probably isn't a milk blister. It may be a “blood blister” from the friction of a shallow latch or a poorly fitting pump or nipple shield. And remember that all white spots aren’t milk blisters. A little clump of hardened milk or a string of thick, fatty milk can plug up your nipple as well, appearing as a little white dot (that’s not covered by skin).
To treat a milk blister, apply moist heat (like a hot washcloth) just before nursing; clear the skin from the milk duct by gently rubbing it with a wet washcloth, pulling gently with clean fingernails, or asking your doctor to open the blister with a sterile needle; nurse frequently (even if it hurts); pump if you can’t nurse; and treat the wound as you would any other nipple crack.