I had a Phal that lived with spots just like that for a long time. All the old growth was affected. Suddenly it developed an advanced fungal infection that tried to take over the whole plant. I had to isolate and prune the poor thing pretty drastically. I am not sure how the plant's current situation is connected to the black spots, but I can't help but think they are linked. I suspect the plant had been living with a fungal infection that only presented itself as those spots, but when the plant had an unusual amount of stress (switching it to S/H medium) it responded by allowing the fungal infection to take over.
This is just my suspicion about what my plant had and gone through, but those spots look VERY familiar to me.
Good luck, I hope this helps some