Maybe Baudelaire is right and you’ve got an infestation… Personally I’ve never got black spots and just wonder if it can’t be something simple like charcoal particles from the filter… I can imagine it being sticky in used filter… But this as a source of it could be easily excluded by checking the filter…
And I agree with no room being 100% sterile and sealed… You can also bring something on your cloth when you visit. And it can fly through air filter…
Anyway, good luck!
I have the problem where holes are showing up on the leaves !
Hi can you show us any pics, it sounds like you may have a pest or something that is eating them ?
A photo would help and a little bit more info about the room they are in etc for anyone to be able to diagnose with any accuracy. 
Did you manage to cut a leaf off with these black spots on to put in a plastic clear box and leave in the room at same temps to see if anything hatches ?
Should’nt take long ie a week for something to hatch.
How are they getting on any more spots appearing or are they the only ones as you posted in the first pic?
It’s not just the black specks - I agree they could be charcoal dust or soot. But together with the white/yellow spots on the leaf, and what looks like striations on the underside, it points to pests. Thrips or mites, or possibly white flies. One thing for sure, if its inorganic dust, it won’t spread and the plants will grow right through it. If not, look out…
If you haven’t done a thorough inspection of the top and undersides of those leaves under magnification, you just don’t know and won’t til it’s rip-and-mulch time.
Essential tool for every grower: Lighted Pocket Microscope
-b420
Yeah, the leaf posted in pic in original post looks damaged definitely…
