I have a blister that has appeared on a scar that that has healed from a scrape on my knee. the incident occurred about a month. blister is 3 days old

Check it out!: Either your doctor, a walk-in clinic or a good pharmacist can help determine infection that needs treatment. It would be foolish to risk a blood infection (from a simple scrape.) also, poorly healing small injuries could indicate something beginning to go awry in your body - blood sugar levels, other biochemicals, etc may cause some lab tests & early detection & easier treatment. Good luck.

Something in it?: Have doctor look at it there may be something in it like dirt or a stitch or may be an abscess present.