When a child wakes up with a large, dark, painful mark on the foot, it is understandable for a parent to worry. A photo might make the spot look like a simple blister, but foot pain, swelling, color changes, and trouble walking are signs that deserve more than a quick guess online.
One possible explanation is a blood blister. These can happen when tiny blood vessels under the skin are damaged while the top layer of skin stays closed. The trapped blood can make the area appear dark red, purple, or almost black.
Common causes include tight shoes, repeated rubbing, a pinching injury, sports activity, or stepping on something without noticing it at the time. Children do not always remember every bump or scrape, especially after a busy day of running, playing, or wearing uncomfortable footwear.
When It May Be More Than a Blister
A dark swollen patch is not always harmless. Infection can cause worsening tenderness, warmth, redness, swelling, or pus. If redness spreads beyond the original area, or if red streaks appear up the foot or leg, that is more concerning.