A blood blister forms when fragile blood vessels rupture under intact skin, trapping dark red or purple fluid in a raised bubble. Often, it’s the result of a slammed finger, tight shoes, or repeated friction from tools or sports equipment. In the mouth, accidental biting, dental work, or sharp appliances can trigger similar blood-filled spots. They may look alarming, but most heal on their own within one to two weeks if left unpopped, gently cleaned, and protected from further pressure.

Still, patterns matter. Frequent or unexplained blood blisters—especially in people with diabetes, circulation problems, or clotting disorders—can hint at deeper issues that shouldn’t be ignored. Watch for warning signs like severe pain, spreading redness, warmth, pus, fever, or slow healing. When a blood blister appears without clear cause, keeps returning, or simply worries you, consulting a healthcare professional is not overreacting; it’s listening carefully to what your body is trying to say.