What is the difference between a lip blister or cold sore?

Blister? / virus: Lip blister could be a burn, having bit your lip and have a salivary gland swollen or possibly some type of chemical burn causing a blister. A cold sore is typically a herpetic virus which will feel feverish, painful and last 10 days to 2 weeks usually.

No difference: Lip blisters and oral cold sores can both be caused by the same thing, either an apthous ulcer or a herpes virus infection. Apthous ulcers are from an unknown cause but commonly associated with stress. Herpes ulcers occur more commonly on the lips. Both tend to last 7-10 days, then resolve spontaneously.