In rec.woodworking, Rick the antique guy wrote:
Thru the years I have discovered some nice natural stain recipes
but I am always searching for more.

A second follow-up: shortly after I posted earlier, I went out to lunch.
Next to the place I ate was shop full of fancy expensive things,
including this indigo stained wood bowl:

https://i.imgur.com/wlmT5QT.jpg

So there's that. Indigo is somewhat difficult to use for fabric, I
don't know how hard it is with wood. The issue is two-fold: a single
dying is usually not dark enough and the color changes after
exposure to oxygen: the color is green before the oxidation; both of
these make "previewing" the work impossible.

Apparently the bowl above was done by a Japanese indigo master.

Elijah
------
had never before seen indigo dyed (stained) wood