Chances are, your bamboo is harboring aphids. They excrete a sticky sweet substance called "honey dew" that attracts and feeds the black mold. Varieties like P. bambusoides with smooth leaves are more prone to them. Over time, they form colonies and prefer the low light, sheltered area in your picture.

Fortunately, aphids and their moldy sidekick on bamboo is mostly a cosmetic problem. The best way I've found is to manage them with a $100 pressure washer from the hardware store. Scrub the branches and leaves with hard blast spray every few weeks until they are under control. Aphids have soft bodies and can be exterminated in large numbers this way. In time, you'll get them under control and your bamboo will drop the old, blackened leaves and make new ones.

The yellow leaves in your picture are probably unrelated and part of the natural seasonal cycle. When the weather heats up, make sure the plants are getting enough water, if the top couple inches of soil are dry...time to water. If you haven't already, your bamboo would benefit from a serving of inorganic time release fertilizer. If nothing better, you can buy Osmocote with the pink top at garden centers and it works great. Hope this all helps :)

Great advice and I have the power washer option ready to go. Thank you for your advice!