Yes, I read that too…so why waiting for hard times, if this is a healthy food?
The water turned kind of yellow-brownish…I think if you boil the acorns, the water might be darker and could surely be used for dyeing.
Discussion
If i remember well, yes, you have to boil the clothes in that water to dye them black. Not 100% sure though but almost.
The tanins are not healthy and as you said, some may remain. It is more like an emergency food.