Take it back to whoever sold it to you, if you can.
A phone is usually blacklisted because it is reported stolen. That may mean actually stolen, or reported stolen as an insurance fraud. It also covers carrier-subsidised phones where someone buys a subsidised handset, sells it then stops paying the contract: the carrier treats this as theft (the buyer effectively bought the phone on a finance agreement then stopped paying) and so blacklists the phone. This last is unlikely for a Nexus 6P though, since they were rarely sold that way anyway and won't have been for several years now. Occasionally phone cloning (itself illegal) results in the phone being blacklisted too.
The point is that phones are blacklisted because of illegal activity, and even if you knew which carrier had blacklisted it they would not remove it from the list because you say you bought it without knowing (if they started to do that it would render blacklisting useless). There is no legal way of doing anything about it, which is why I say take it back if you can.