Nothing bad is going to happen to the patient because of this. His BP was "a little high" on the auto cuff, so it's almost impossible that you'd have found a BP that was in the scary high zone (think 220/120). And he had no symptoms, and labs normal so no evidence of end organ damage. So, no worries there.
The good news is that you seem to feel terrible about this. And that's good, because the most important thing when you're in medical training - whether a student or a resident - is trust with your preceptors. Once you start lying about things, or omitting things, or saying "it was normal" when you didn't do it / look for it at all -- it's only a matter of time before it does matter. And once those people around you don't trust you, you'll find it's very difficult to proceed.
No one expects students or residents to know everything. Just admit when you don't know, or something seems off, or you're not certain. Your preceptor is there to help you. And you won't improve unless someone shows you and helps.
Interestingly, your post which is entitled "Ethical question" doesn't actually pose the ethical question your action triggers: should you speak to your preceptor and admit the issue? Not because anything different will happen with the patient, but because it's your ethical duty to be honest with your preceptor. Like most ethical questions, there isn't a right answer. There are options, and you need to live with the consequences. You could talk to your preceptor about it. They may thank you for your honesty and tell you to ask them the next time you have a problem. They may kick you out of their office and refuse to work with you again (very unlikely). Either way, it's off your chest and you move forward with your life. Or, you keep quiet about it and there are no external consequences -- but you have to live with this. No right answer.
