Hello from JustAnswer.
It is true that herpes is a common cause of a blister around the anus. And if the blister recurs in this location intermittently, that would very likely be herpes.
As for whether you should reschedule, that is a personal decision. There is not reason to avoid doing the colonoscopy from the perspective of the doctor, as there is no risk of transmission of herpes to the doctor, since the doctor will be wearing protective equipment, such as gloves. There also is no risk of transmission of herpes via the colonoscope, since it is sterilized between patients.
The most common reason for consideration of rescheduling the colonoscopy would be if there is pain or discomfort associated with the blister, as the introduction of the colonoscope may exacerbate the pain or discomfort. As for embarrassment, there is no reason for embarrassment, as doctors regularly see patients with a wide variety of infections. And if you want to receive a definitive diagnosis and prescription for an antiviral medicine, a visit to the doctor is the usual method by which this is done.
So, if there is pain or discomfort associated with the blister, it would be reasonable to reschedule the procedure. If you are significantly embarrassed, it would be OK to reschedule, but doctors would usually encourage people to seek proper medical care.
If I can provide any additional information, please let me know.