Does it rub off?

No it does not rub off. I thought of the same thing. The underside has no markings like the top, but it does have a kind of "shadow" in the area where the markings on the top correspond.

And this morning I noticed more of this in in my lilac stand.

Hmmm. I don't like that. Fungus seems to be rampant this year all across the country. Bulb lilies getting botrytis, roses balling, I even got mildew on a nandina! How can that happen? My guess is some sort of fungus, but gee, I've never seen anything that looks like that before. I hope it disappears right away.
Renee

The leaf doesn't look at all yellowed, which one might expect with some sort of fungal ick. A little further into dormant season, you might want to dig up the clump and take a look at your rhizomes.

Kay