I wasn't comparing lances to the black ship, admiral, I was simply stating the fact that the same methods of argumentation came up in the sm lance thread.
We both have our opinions, and there is no hard fluff that says what armor a black ship has. Here is my reasoning.
Why do Imperial warships have 6+ prow armor? Because the Imperial method of warfare is to close with the enemy head on, firing torpedo volleys, until they get close enough for broadsides. That prow armor is an integral part of their strategy, to take the head on damage and weather it.
A Blackship is a treasure ship, one of the singularly most valuable ships in the galaxy. It is extremely well defended, with only cruiser offensive weaponry. It isn't going to be part of a battle line. Its going to retreat from any serious engagement under the power of its escort fleet.
It will rarely be pointing its prow towards any threat, its far too valuable for that. 6+ armor therefore I consider to be a no-brainer for this ship.
And I hold that the reason of 'it isn't defended as best as possible because they are afraid it would defect' to be a shaky idea. Thats not how the Imperium functions. They prevent defection with multiple redundant restrictions and chains of command, not by weakening their ships.
If a blackship wanted to defect, it could do it much more easily than trying to take on its escorting fleet. Armor matters little in that scenario.