All Knives-Swords-Axes and Cutting Tools have been Convex Ground and Sharpened from the Bronze Age to about 1900. The Advent of Automated Grinding during the Industrial Revolution saw the first V-Bevel methods of Sharpening knives. This was necessary because there are no machines--even today--that can do a proper Convex Grind or Convex Edge.
Convex Grinds and Edges are all Hand Done.
The Advantage of the Convex Edge is that it supports the Carbides on the Cutting Edge by an almost 400% advantage to the -now common- V Bevel.
Convex Edges hold their level of sharpness far longer than a V bevel and are 4 times tougher. They are also deceptively sharp.
The Thick Edge Spine holds the carbides on the edge to such a longer degree that the knife stays sharper far longer and will still cut long after a conventional V bevel has gone stone dull.
Convex Edges are also the Easiest to Maintain and restore.
A quick strop and they are right back to Razor Sharpness.
KEEPING YOUR BLACKJACK EDGE
DON'T use a stone. Stones make flat places and your edge is convex. Just stroke the edge --- only the edge --- backwards on cardboard, leather, crocus cloth or any other MILD abrasive every few days and that is it. Yes --- just lay the knife flat, lift the back of the blade until just the edge touches the surface, and pull it firmly backwards, with down pressure, stroking the whole length of the edge. Repeat on the other side. Stroke point-to-handle OR handle-to-point, whichever works for you.
Edge gets flats or nicks? Easy. Get wet-or-dry sandpaper in 320, 600 and 1200-grit, lay it on a mouse pad or other yielding surface. Stroke your blade backwards in the same stropping mode, first on 320 grit, then 600, then 1200, and repeat as necessary to eliminate the damage. Finish on cardboard (or leather, etc.) and you will have your beautiful convex edge back.