It could be a blood blister, especially if it looks like a dark red, purple, or black raised spot. A blood blister usually happens when tiny blood vessels break under the skin while the top layer stays closed, often from friction, pressure, tight shoes, pinching, or a small unnoticed injury. Cleveland Clinic describes blood blisters as commonly caused by pressure, pinching, or friction, and they can appear on the feet.

But the part that worries me is this: she is in pain and can’t move her foot. That should not be treated like a simple harmless blister at home. Severe pain, trouble walking, swelling, warmth, spreading redness, pus, fever, or red streaks can point to infection, injury, inflammation, or something that needs medical care. Mayo Clinic says infected blisters can show increasing pain, warmth, pus, or spreading skin color changes.

So the safest explanation is: it may be a blood blister, but because she has pain and cannot move her foot normally, she should be checked by a doctor or urgent care. Seattle Children’s guidance says a child with a new limp, abnormal walking, a bright red skin area, or unclear pain should be evaluated.

Until she is seen, do not pop it. Keep pressure off the foot, let her rest, elevate it, use a cold compress wrapped in cloth for short periods, and keep the area clean and protected. Go urgently if the spot is spreading fast, the foot is hot or very swollen, she has fever, pus, red streaks, numbness, blue or cold toes, or the pain is getting worse. Cellulitis and other infections can spread quickly and should be treated early.