Eames DCM touch-up question  

You're correct that the black finish is more like a paint than dye-stained wood. What it is, exactly, is proprietary, but I suspect it's a pigmented lacquer.

I rather like the way these look as the finish wears and exposes the underlying ash veneer. I wouldn't experiment on them. It's too easy to make things much worse.

You can easily touch up wear on edges with a marker but don't try that on the seat, as you'll never match the color or the sheen-- the whole seat has to be sprayed to be make it even. You might put a simple hard wax on it for some protection and luster but I wouldn't sweat it too much. No india ink, yikes!

Marker? You mean like a Sharpie or old-fashion Marks-A-Lot permanent type?

Why would a "permanent" marker be OK but not India ink?

You can actually order the ebony dye directly from Herman Miller.

I mean furniture touch-up markers. The Herman Miller touch-up kit is probably a marker and a spray can. That's what it is from everybody else, anyway.