What is a blacklisted iPhone?
Blacklisted iPhones are iPhones whose IMEI have been blacklisted on a shared carrier database in a country thereby making it impossible for the iPhone to be activated on any carrier in that country.
Why do carriers blacklist iPhones?
iPhones get blacklisted by carriers so they can no more be used on any carrier, this is mostly as a result of the imei of the iPhone being reported as stolen or lost, unpaid bills or insurance fraud by the owner.
Can a Blacklisted iPhone work overseas?
Blacklisted iPhones whose imei have being added to a shared carrier database will work in other countries not sharing same database. Most of those database are shared between carriers in a country their by Blacklisted iPhones will work in other countries apart from the country it was blacklisted.
How to unlock blacklisted iPhone
If you bought a blacklisted iPhone on Facebook, Craigslist, Offerup etc and you find out its blacklisted, you can unlock the blacklisted iPhone by using an unlocking chip like Gevey chip, Rsim chip etc. There are also many services out there that claims to clean blacklisted iPhones.
Apple blacklisted iPhones
Yes, most blacklisting is done by the carrier and not by apple but recently apple has started backlisting iPhones. iPhones blacklisted by apple cannot be activated anywhere in the world, this is a global blacklist as the iPhone will not activate and will just be a paper weight unlike carrier blacklisted iPhones that can still be used as iPod.
How to check if iPhone is Blacklisted
To check if an iPhone has been blacklisted by a carrier, you will need to run an imei check. Imei can be found on the about page of the iPhone or by dialing *#06# on the iPhone. There are sites that runs free imei check like iUnlocker.net
It is advised to always run an imei check on iPhones before buying a pre-owned iPhone.