https://cold-hardy.com/bamboo-fungal-infection/Does it look something like this?
I did not come to a conclusion regarding this issue. Spraying aspirin dissolved in water over the leaves did seem to help, but after a while, the problem became even more evident. It always happened inside, never outside in the open. I was using LED diode grow lamps. They are purple and plants just love them. Perhaps bamboos are more delicate when it comes to light spectrum and they started blackening.
I guess if the damage is the same, we can try figuring out the problem considering your growing conditions and comparing them with mine.