The best blackjacks used whalebone as the spring device; this allowed the thing to jump back into the ready to use position; nearly all cops carried them when I was young. I was never hit by one but I did have a couple of them used as a threat by Dallas cops. Most of my contemporaries had similar experiences; it was all about intimidation in those days. I did know plain old people who carried and reputedly used them, but I never saw it done. I would imagine a blow on the head from one would drop the average guy in his tracks. As the OP mentioned, they used to be commonly available in pawn shops. As did cheap pistols, switchblade knives, and brass knuckles.
My employer once had a security guard who was a retired police officer. He carried a blackjack. I asked him why he carried that, rather than a baton.
He said, “I can knock a guy’s teeth down his throat . . . but I don’t break bones, like they do with those batons!”
Personally, I would rather have a broken arm than broken teeth, but he thought he was being more humane.
<shrug> I worked armed security, hence the crack about tonfa as I went through the training, and I was a domina for a while, hence the experience with other forms of tool.
Nooooooo! don’t be calm and rational about it, that just makes it worse somehow.