A milk blister, or bleb, is a white spot on the surface of the nipple that looks milk filled.  You may feel sharp pain or aching throb with a milk blister.  A clog can be related to a present blister, but not always.  Clogs form when milk is not efficiently drained from the breasts.  When this occurs, the milk will back up in the ducts forming a sensitive lump.  You may feel sharp or dull pain with a clog.  Clogs can appear near the nipple or under the areola, anywhere in the breast, and in your underarms.

Clogs and blisters can appear when one goes too long without nursing or pumping, missed feedings, excessive pressure in the breast from a bra or top, and lack of removing sufficient milk from the breast due to unrecommended timed feedings or issues with baby's latch.

The best thing to do when you are experiencing a clog or blister is to nurse your baby as often as you can.  The more you nurse the more opportunities you're giving your baby to release the clog / blister.  There are several things you can try to help in assisting baby in doing that:

Massaging the lump with an electric toothbrush or vibrating sex toy can help loosen the clog.  If you prefer to hand massage it's recommended you use the side of your hand to apply pressure rather than poking the area directly.  You can do this while taking a hot shower or after applying a hot compress.

Nursing hunched over baby on your hands and knees with the assistance of gravity can help move stubborn clogs along. 

As recommended by KellyMom.com, taking sunflower lecithin daily can help relieve and prevent future clogs / blisters.  The recommended dose is 3600-4800 mg lecithin per day, or 1 capsule (1200 milligram) 3-4 times per day.  If you notice you have not had any blockage after 1-2 weeks then you can reduce the dosage by one capsule, reducing it one capsule at a time every 2 weeks if appropriate.  You may need to continue taking 1-2 capsules a day if stopping the lecithin leads to additional plugged ducts.  https://kellymom.com/nutrition/vitamins/lecithin/

Pumping can help as well, but it's important to make sure baby has been fed first and that you're not pumping too much too close to early postpartum.  Doing so could lead to an oversupply issue.

If a milk blister / bleb is present you may see a thin layer of skin over the duct opening (where the milk exits the nipple).  This is not to be mistaken for a montgomery gland, which are small oil-producing glands that provide lubrication altering the pH of the skin, discouraging the growth of bacteria on the skin of the nipple and areola.  Taking a sterilized needle you can rupture that layer by scraping it softly.  Try to avoid poking or stabbing it directly.  It is best to attempt this after getting out of the shower or taking a bath.  If a shower or a bath is not possible at that time, you can soak a diaper, fill a shot glass, or suction a Haakaa milk catcher with warm water and epsom salt making sure your nipple is soaked in the solution.  Rubbing the blister with a wet microfiber towel or washcloth when the skin is moisten can also help safely rupture that top layer.  Applying thick nipple cream between nursing sessions can help keep the tissue moist, making it easier for baby to draw out the clog / blister while suckling.
 
Some blisters and clogs can be stubborn and take a few days to drain out.  Do not be discouraged by this and continue your remedies at every feeding.  It’s perfectly normal to feel pain or sensitivity after it has been cleared.

It is important to stay diligent in trying to clear these issues as leaving clogs sitting for too long can put you at risk for mastitis, or even worse, developing an abscess.  If you believe you have mastitis, which includes body aches and chills accompanied by a fever of 102 for 24 hours, it's recommended you call your doctor so they may determine if antibiotics are needed.