The word official, examined
Every clone calls itself official. The word is only useful if it points at something checkable, so here is what it can point at.
Search results for this beat are full of pages announcing themselves as official. The word appears on the real thing and on every imitation of it, which means encountering it tells you nothing. It is worth asking what it could mean that would be useful.
What it cannot mean
There is no authority here. No registrar, no certificate body, no organisation that grants or revokes the status. On clearnet, official has thin but real backing, since somebody registered the domain and a certificate authority made a limited claim about it. None of that exists for a hidden service. A site calling itself official is making a claim about itself with nothing behind it.
It also cannot mean first, best known, or most linked. Those are properties of reputation, and reputation attaches to strings that can be copied. A retired address that half the internet still links to is well known and no longer official in any sense.
What it can mean
Exactly one thing that survives scrutiny. That the party answering holds the private key which the community has come to associate with the platform, and can demonstrate it by signing something. That claim is checkable, does not depend on trusting anyone's say so, and cannot be faked by someone who does not hold the key.
Everything else people mean by official reduces either to this or to a feeling. The feeling is the thing attackers manufacture, and the check is the thing they cannot.
The circularity, and the way out
There is an obvious problem. Verification proves the signer holds a key, but how do you know that key is the right key? A page that hands you both a key and something signed by it has proved nothing except that it is internally consistent, which any forger achieves for free.
The way out is not clever, it is just work done once. Get the fingerprint from several places that would have to be compromised separately to agree on a wrong answer. Check it carefully, the whole string rather than the ends. Store it somewhere you control. After that the question never arises again, and every future check is mechanical.
The one situation worth extra care is a message announcing the fingerprint itself has changed. It is genuinely hard, because anyone holding the old key can sign a handover with it. No single message settles it. What settles it is consistency across independent places over time, which is why the correct response to a key rotation notice is to slow down and wait, never to act inside whatever deadline the message sets.
Practical reading
When you see official on a page, mentally replace it with the words this page says about itself. Then ask what the page offers that you could check. If the answer is a signature and a fingerprint you can compare, the word was doing real work. If the answer is the word itself, in a large font, next to an address it wants you to use, you have learned something about the page but not the thing it was trying to tell you.
Verified working Nexus Market mirrors
Three v3 onion addresses currently serving the production market, signed under PGP fingerprint 0x7F2A0A9D. Use the Copy buttons.