A milk blister, or blocked nipple pore, occurs when a tiny bit of skin overgrows a milk duct opening and milk backs up behind it. A second type of milk blister appearing as a white spot on the nipple may be caused by an obstruction within the milk duct, as opposed to skin covering the milk duct. A completely different type of blister is caused by by friction from baby’s improper latch or sucking or from the use of a badly fitting nipple shield or flange.