I’m really glad you shared the photo — this looks serious and painful, especially for a child 😟
From what I can see, it looks very similar to a blood blister (a blister filled with blood instead of clear fluid). These can become large and dark when pressure or trauma happens while sleeping (for example from tight shoes the day before, friction, or a small injury that wasn’t noticed).
However, because of the size, dark color, and strong pain, it could also be something more serious like:
or in rare cases, a reaction to an insect bite that became swollen overnight
The pressure inside the blister/swelling can press on the nerves in the sole of the foot, which makes walking extremely painful. Kids often refuse to move the foot when the pressure is strong.
Please don’t try to pop it at home — because if it is infected, opening it can make it much worse.
give treatment to stop the pain and swelling
Is the spot hard or soft when you touch around it?
I’ll help you figure out how urgent it is based on that.
