Found an iPhone 6, but it has a password lock?

It's been a month and no one is calling to claim it. If I unlock and factory reset it. Will I be able to use it in the same country? Or will the phone manufacturer ID be on a blacklist?

Your best way is to take it to an Apple store where they will trace the IEMI number to its correct owner and also be able to let the original owner where it is for data retrieval and it is probably system locked and even unblocking it will not work unless Apple do so and they shouldn't do it for anyone other than the original owner

You wouldn't be able to do anything since it has Activation Lock.

You won't be able to "unlock and factory reset it". Turn it in to law enforcement, any nearby cell phone service provider's store or an Apple Store.

Your thought process is currently similar to "I found a car in a parking lot. If I rekey it, will I be able to use it?" In case it is not obvious, the answer is "No".

The top left of the lockscreen has the carrier identifier. Take it to the mall and find the provider's kiosk or store - I fail to see why the person that lost it has to call it in order for you to consider giving it back.

They'll get the phone back to the owner. This is how I got mine back when I lost it, and you should hope this is how you'd get yours back.

The IMEI will be reported, and it has an activation lock.

Unless you know how to flash, change its IMEI and willing to risk bricking the iPhone, you won't be able to use it. Just hand it in to your local police station so they can trace the original owner and hand it back to him/her.

If you hold down the home button to activate Siri and say "Siri who owns this iPhone?", Siri will say "I believe this iPhone belongs to (Siri will state the person's first name)" and will pop up the owner's info on the screen so you can return it to its rightful owner.