How accurate is a blood test for determining if you have cold sores?

Not very helpful: Cold sores are found on physical exam. Sometimes testing for viral infection s can give a specific cause, but doesn't change the treatments very much. See your doctor.

Generally not needed: Cold sore can generally be diagnosed clinically, without need for blood test, especially if you have cold sore before. Blood test only tells if you have had infection with HSV 1, not if a given sore is due to HSV.