Looks like a small wooden baton, which is weird, as a blackjack is a spineless leather baton filled with lead shot (more like a kosh). Very heavy, with a large contact area to make a knockout more likely than an open cranial fracture.
That thing in Garrett's paw looks like a miniature club. You don't knock people out with clubs; you tend to beat them to death with them. While knockouts would be common, they would not be as common as a concussed but conscious enemy, holding his wounded head with one hand, the other on the nearest wall for support.