Hello, Thanks for your query. Oral blood blisters can be caused by a condition called angina bullosa hemorrhagica (ABH), in which acute, benign oral mucosal blood blisters can suddenly appear in the oral cavity. These may be cause by mild trauma to the mouth, perhaps by eating hot food. These blisters usually rupture quickly and heal without further discomfort. They are not usually serious, except in rare cases when large blisters don t rupture spontaneously and can cause airway obstruction. The average size of blood blisters due to ABH are a half centimeter to 2 centimeters, and can appear on the palate as well as the tongue. Blood blisters on the lips and gums may be due to a different condition called Immune Thromboctyopenic Purpura, an illness in which low platelet levels lead to abnormal bleeding. Contact your doctor about any changes in your health that may indicate underlying issues. I do hope that you have found something helpful and I will be glad to answer any further query. Take care