Well I mean if you had to choose between the two, I’d say Black Knight is easily better than Queen of Hearts without question. It’s like the distinction between Cyprian and Boril: they do the same thing basically, but one is generally better than the other.
There are probably corner cases where Queen is better, but those would be rare.
Just the raw stats alone make Black Knight a better tank or flank. Queen’s buff is just more consistent than Black Knight. Yes, Black Knight could proc multiple times and take like 3 or 4 damage off a sniper barrage, or he could proc 0 times and get burned down by combo. Queen is more predictable in her behavior, and her INDIVIDUAL damage output is higher than Black Knight due to having a higher attack score and her minions contributing to the fight if they’re allowed to pile up as well as being a 3 hit without “minor damage” tacked on. This could be more tactically useful depending on the situation, but I don’t know that that outweighs everything else that Black Knight brings to the table. Finally, if you pair Queen with one of those other rare minion creators like Delilah, Santa, or Red Hood, the minion can stay around for quite a while which can be a pain in the butt, but Black Knight’s a 3 turns and done which is more unpredictable, especially if the AI is piloting him in defense.
Black Knight helps the team in multiple ways whereas Queen doesn’t do that… like at all. Queen you can burn out the minion with tile damage if the tile gods favor you. Black Knight, you either have to have a dispel ready to fire or wait it out, neither of which are great options. Black Knight is far sturdier given his defense stat, and his massive berserker buff is a great boon to any team.
So I’d say in general that Black Knight is better than the Queen, and if you’re one of those people so lucky to have both, Black Knight is clearly the better option in all but a few very corner case scenarios.
But they’re both pretty darn good.