If the black powder is growing on the rhizome, it could be a fungus/mold. I've only seen green mold on some I bought via mail.

Deer will eat foliage if hungry ~~ not their first choice like roses.

Mold comes in all colors. I don't know where you live but if you are living where all the rains and humidity are, it is very possible it is a mold. If the powder seems 'stuck' to the rhizome it is even more likely it is a mold. If you think that might be the cause, You can spray with a fungicide.
Laetitia