You have not mentioned whether this blister is inside the lips on the mucosa or on the outside lips. If it is inside the lip then possibility of this being due to canker sore or mucocele are more because herpes usually do not produce any lesions inside the lips.
Canker sores are small shallow ulcers that appear in the mouth and often make eating and talking uncomfortable. They are generally caused by Stress, tissue injury, eating certain foods – including citrus or acidic fruits and vegetables or underlying health condition, such as an impaired immune system; nutritional problems, such as vitamin B-12, zinc, folic acid, or iron deficiency; and gastrointestinal tract disease, such as Celiac disease and Crohn's disease.
Herpes simplex type I blisters or Cold sores or fever blisters are one and the same thing. Unlike canker sores, cold sores are caused by a virus and are extremely contagious. Also, cold sores typically appear outside the mouth – usually, under the nose, around the lips, or under the chin while canker sores occur inside the mouth.
I sincerely hope that helps. Take care and regards.