The photo you sent is alarming, and you are right to be worried. A dark, swollen, intensely painful spot on a child’s foot overnight is never something to ignore. It might be “just” a blood blister — or the first sign of something far more dangerous. The difference between waiting and acting today could det…
From your description and the photo, this could be a large blood blister from pressure or unnoticed trauma, but the size, color, and pain also raise concern for infection or a severe reaction. Do NOT pop or cut it at home. That can introduce bacteria, worsen a hidden infection, and make walking even more painful.
Check gently: is the area soft and squishy like a fluid-filled blister, or firm, hot, and very tender even to light touch? If it’s hard, hot, rapidly swelling, streaky, your child has fever, or refuses to bear any weight, go to urgent care or the ER today. If it’s soft, not rapidly spreading, and your child is otherwise well, you can still call your pediatrician or an urgent care now for same-day evaluation and pain relief, so they can safely drain or treat it and stop the swelling and pain.