May I suggest a simpler and more benign explanation for your tar spots?
They may well be insect poop.
Several years ago my orchid collection (grown under lights) seem to be
suffering from a similar affliction: dark, tarry spots on the leaves.
I, too, worried about it. Then I noticed a few tar spots on the light
tubes themselves. The only explanation, of course, was insect droppings.
I never saw the critter that made them. It seemed to be a night creature
that had definite preferences as to where it would hang out and that
did no damage to the plants that I could tell. It also survived
just one season, if I remember right.
Try scraping the spots off. If the tissue beneath them looks healthy, I
wouldn't worry too much about it.
Brooke