flat white painless itchless blister on back of penis
Hello and welcome to ask a doctor service.
I have viewed the image and gone through your query and here is my advice.
Seems like penile papules. They are a normal finding. Nothing to worry.
Hope I have answered your query. Let me know if you need any more assistance.
Possibility of genital herpes
This one is a lot better. Could be a herpes blister. How long since you have noticed it? Any history of risky unprotected sex or promiscuity?
Let me know if you need any more assistance