[bactrus]

Think they're mites Ken. Black stuff? I was thinking of red spider mites or aphids. Red spider mites cause those indentationl on leaves. Best way to get rid of them is to keep the plant wet as they hate water. Mist two 3 times a day regularly. This method is chemical free as my orchid sifus thought me when my Phalaenopsis became infected. Bottom line, red spider mites hate moist damp condition.

Apihds, easy to get rid off. Just spray H20 (by Fumakila) on it. The black spots on the bottom leaves could be suger residue. Aphids suck plant juice then extrete (yes their feaces) sweet droppings for ants to feed on in exchange for protection. Absence of ants would mean these sweet droplets dropping to the bottom, in your case the bottom leaves where it cause balck spots.

Good idea to check other plants too.