I have the same problem and think it is fungus. Spraying alcohol and rinsing off with water did not help. Sorry that I have no answer.
---------- Post added at 08:18 AM ---------- Previous post was at 07:49 AM ----------
My orchid book identifies this as Sooty mold: a black fungus and excreted by aphids, etc. It does not harm a plant but is unsightly. Mine does not wipe off so it may be something different.
---------- Post added at 08:31 AM ---------- Previous post was at 08:18 AM ----------
Lars Kurth posted this link on his thread about pests ...
Orchid Diseases. He is very helpful and generous with his orchid knowledgr