What happens to an iPhone after it’s been blacklisted?

Long story short, I lent an old iPhone temporarily to someone as their android phone had apparently been “hacked”.

Time had passed and my loaner iPhone had never been used. I had asked for the phone back. The person just disappeared. I have tried contacting this person on numerous occasions and in different ways (text, call, messenger) to get it back, but never heard from them.

So today I contact my service provider and I decided to have the phone blacklisted.

I am now wondering what happens to the phone/line if the person eventually ended up using it or maybe even sold it?

I know that a phone that is on the blacklist can not be activated, but what of it already is activated? What happens?