Apple

Buy a new phone and don’t get scammed.

If it works now, keep using it until it doesn’t. Not all countries talk to one another about blacklisted IMEIs and some take a long time to update. Be ready to buy a new phone. Keep this one as a music player.

From what I can tell the IMEI databases are not international - the phone will not work in the country of origin (or maybe in North America at all) but it should still be fine in Europe. As long as it is fully unlocked it sounds like it won't be an issue except if you plan to travel with it.

An IMEI locked phone or one that is blacklisted is only blacklisted from being able to connect to local carriers (again, this is as far as I know, there could be specifics i'm unaware of!) but otherwise the device will work just fine on Wifi and for everything else that doesn't require mobile data / LTE / 5G - as long as it has been signed out of the previous owners icloud account and Find My to disable activation lock - it's essentially like not having a SIM in the device.

I'm not familiar with the idea of blacklist phone. If you can use it I'll do you know it's blacklist? Is there a message saying that

Probably blacklisted only in the US. Usually it’s carrier thingy. If you can use it, use until you can’t.