The idea that opening a package implies that you know its contents is yet another dubious legal fiction used to transmute the rule of law into the rule of gangs.
Opening a package addressed to you with illegal items can leave you open to possession charges if you don't immediately report the items.
There is some grey area involving whether or not you should know they are illegal, and is entirely up for interpretation - being gifted a bouquet of poppies is still different than holding on to a pillbox of opiates though.
By doing anything other than destroying it themselves, the recipient is engaging in conspiracy and distribution. You're also assuming they can quickly identify what the substance is, and also immediately understand every applicable law. Of course we're out in the weeds here, since we've accepted the premise that simply possessing a substance (with no mens rea) can be a criminal act!
In an environment of legal contradictions you're never in the clear - you can only kowtow to the biggest gang and hope they're merciful. But let us not corrupt our own sanity by internalizing their farcical justifications.
You can claim you didn't know there were drugs in the package if you don't open it and don't sign for it.