They aren’t supposed to be knocked out, they can’t become your fighter, if I am correct. Several npc can’t be knocked, like the prowlers, those can be hired in a tavern so now and then.
In the past they have been able to be knocked out. I used to have a bunch of them as thralls. That’s why they have the little white line above their health when you hit them with the club. Unfortunately, the dial on knocking them out has been turned way up. The only ones you cannot knock out (or at least could not in the past) were the jailors and the one skull minibosses. Everything else was fair game. The Corsairs have the highest rate for t4 taskmasters and crafters. The t3 fighters were good to start/mid level with.
regular t1-3 Black Corsair can be knocked out. All 1Skull boss Corsairs cannot be knocked out. Years ago, Black Corsair Elite from Pirate bay in Exile Lands was capturable but it was a very very long process but not anymore. ( I say this in case you see anyone with them used).
That would be incorrect at the moment. Just tried it today on at Archer TIII and they were immune to steel truncheon. So still not fixed. At least, not on #6443.
Sorry for the confusion. I thought this post was all about the Black Corsairs on Siptah. Not even sure where the Black Corsairs are in the Exiled Lands.
I also came to write about this problem. There are many camps with Black Corsairs on the Island of Siptah. There are not only mini-bosses that cannot be stunned/tamed. There are also many artisans, sorcerers and fighters who could previously be stunned and tamed, but now they cannot. I hope this problem will be fixed as soon as possible.