A boil that's already drained and is shrinking, with no redness, warmth, or fever, is usually healing on its own. The dark mark it left behind is common after a boil settles and it usually fades with time.
Next Steps
Keep the area clean and dry, and avoid touching or picking at it while the mark fades. If it comes back, grows again, or the mark hasn't faded in a few weeks, get it examined in person.
Health Tips
See a doctor sooner if you notice new pain, swelling, warmth, or fever.