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:

a deep foot abscess

a severe infected blister

or in rare cases, a reaction to an insect bite that became swollen overnight

Why she can’t move her foot

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.

What you should do now (important)

Please don’t try to pop it at home — because if it is infected, opening it can make it much worse.

You should take her today to:

a doctor

a pediatrician

or an urgent care clinic

They may need to:

check if there’s infection inside

safely drain it (if necessary)

give treatment to stop the pain and swelling

Go urgently if you notice any of this

fever

the area getting bigger

redness spreading around it

she cannot stand at all

the skin becoming more black/purple

If you want, tell me:

How old is your daughter?

Did she wear tight shoes yesterday?

Is the spot hard or soft when you touch around it?

I’ll help you figure out how urgent it is based on that.