Anything I can think of would involve disassembling the wheel. For example zinc electroplating. You could paint them black, but I've seen paint flaking off which looks worse.

So, I'd pick out your worst set of wheels. Check the length of the spokes (usually through the base of the nipple and slot). Disassemble one. Measure the spokes and buy all new stainless spokes. Also take the time to do a good job polishing the hub and rim. Then rebuild.

The only way to learn to build a wheel is by building a wheel.

Oh...
Also, check for brake track wear, and overall roundness, trueness. No sense investing time and money in a wheel with a worn out rim.