I actually had a similar problem and I couldn’t find a solution online. My dischidia wasn’t the aerial as shown but I hope the solution to this darkening is still the same. When I did a little Googling around, most articles and blogs (not many forums, unfortunately) pointed out over watering as the main cause of this darkening. So I cut back on the watering, from 4-5 times a week to only twice.
I waited for like three weeks to see change, but nah, nothing happened. The blackening didn’t go away. I decided to find the next solution. I found an amazing article (forgot the link but I’ll link later) that was detailed enough to explain what to do if cutting down on watering doesn’t work.
Apparently, when your dischidia becomes black, especially on the lower leaves while the upper leaves stay green, it’s probably due to a fungus attack. And get this… such an attack is due to over watering! Yeah, that’s right.
Because the attack has already started and is fast spreading, just like mine did, the best solution is to have it re-potted. Which means carefully removing the dischidia from its current pot and replanting it on a new, dry soil. You’ll also need to cut down the black, dead leaves and stems.
I know, it’s a whole process. Very tedious actually. And like any other normal gardener, I’m always skeptical of online “advice” if it doesn’t come from a forum where other fellow gardeners can green light or give it a thumbs down. But hey, I was desperate and so I gave it a go. Long story short, the re-potting worked like a charm. The trick is to make sure to water your dischidia is on a modicum level so it doesn’t become vulnerable to fungus. Go ahead and try re-potting and see what happens.
Let me check and see if I can find the article on this. It actually had an amazing step-by-step breakdown on how to go about the re-potting. It also had other alternative solutions on how to protect the dischidia from fungus attacks.