When refinishing posh kitchens using THAT brand of posh paint, which looks nice but shows up every mark, I often recommend a coat of eggshell acrylic varnish. This improves the scuff-resistance of the surface. I use Craig & Rose which whilst a bit pricey is good stuff.
Two things are important. Water-based coatings don't like uncured oil, so I always leave a minimum of a week before varnishing (for a gloss, which contains more oil, maybe longer). The other thing is that water-based coatings generally like to have a mechanical bond, so rubbing down the surface is essential.
You could use an oil based varnish, but again rubbing down first. It will yellow over time but that shouldn't matter over black. I think you would need a "long" oil varnish rather than a "short" resin type varnish, then it would remain flexible over the gloss. I think "yacht" varnish is "long".