IMEI blacklisted - iPhone still works?

I got an iPhone 7 as a gift over a year ago. This phone was brand new when I got it, in packaging and untouched.

My reason for posting is because it has since come to my attention that this IMEI is blacklisted. Although, my phone works perfectly.

How could this device be lost or stolen if it was new when I got it?
How does it work perfectly if it is a blacklisted phone?

It's been on my mind and I can't get more info off the gift-giver as we're no longer in touch. Any help would be appreciated.

You know nothing about the phone other than it was given to you. It may have be stolen new in the box, along with several other phones. It may have been part of a BOGO promotion, where you keep two lines open for X amount of time, or the gift giver skipped out on paying off the phone. Really doesn't matter, it's blacklisted, and you don't have the cash to pay off the debt or the information to clear up any report of it being lost or stolen, because you don't know how it got into the gift givers hands, before being gifted to you.