The keys don't really look proper unless they have legends. I have tried a few techniques so far.

This is a picture of two interesting tests.For the reference, the key pitch here is 10mm. So the key tops are roughly 8mm across.

The bottom one has the legends embossed in the design, then printed normally in white PLA on Ender2 with a regular 0.4mm nozzle (although I think I had extrusion width set to 0.3 for this test). Then I sprayed it over with a black paint and sanded down the tops. It's definitely usable, but it's brutally ugly.

The top one in yellow is more interesting and that's the method I'm counting on. The legends are printed on regular glossy paper, the kind you'd use for toner transfer PCB making. The keyboard and the paper are then sprayed with alkyd-based lacquer. A few moments after spraying, so that it's already not too runny but not yet hard, the paper is placed on the plastic, toner side down, adjusted (as you can see adjusting it is trickier than it seemed), and let dry overnight.

Then it's as if it were a PCB. The paper is soaked wet, peeled off, the remaining bits of paper are brushed off using an old toothbrush. There will still be some visible bits when the water dries, but here comes the magick with a k of another coat of transparent lacquer. It all becomes transparent and it looks amazing.

I make a point of using an alkyd-based lacquer because I also tried an acrylic variant and it uses a base that immediately dissolves printer toner.

This is really nice so far, but what about those brown keys? Black legends will not look good on them.