I just don't see any benefit of telling my wife about something petty from before we were even married (or engaged)

It's not petty if your wife would consider divorcing you over it. Then, it's major. And if it's petty, then just tell your wife because it will be much better if you're the one coming clean to her, rather than her hearing it from a friend.

Here's the thing, you know it's not a 'petty thing,' which is why you are choosing to withhold it from your wife. You fear the consequences of your actions and are willing to go to great lengths to hide the information. While mistakes happen and people have momentary moments of judgement, doing what you are doing now just straight up makes you a shitty person.

If kissing some chick 7 years ago is THAT important to your wife, then it's really shitty that you are keeping that information from her. It's really shitty that you are letting her stay in a marriage she hasn't chosen using deception (because she chose a marriage with you based on faulty facts). It's really shitty that now, rather than just come clean and let her have the facts so she can decide what is best for her, you are actually deliberately making sure she doesn't get that choice, going so far as to blackmail your friend.

But, like I said, it's your choice. If you want to do this the cowardly and selfish way, go for it. Just don't pretend it's a "petty thing." If it was a petty thing, telling your wife shouldn't be a problem.

Personally I hope your wife finds out not because I think it will break apart your marriage (it probably won't) but that it will force you to actually face some consequences. You're putting in all this effort to hide it from her that I don't see why it's not better to just tell her and put in the same amount of effort in rebuilding trust and making your relationship stronger and better than it is now. But that's just me.