The round tiny black spots found on siding are spores called Sphaerobolus stellatus, also named artillery fungus blast or shotgun blast.
WHERE DOES IT COME FROM
You ready? your bark mulch. It is the decomposition of the mulch. They can be found on top of the mulch. Look for light brown/orange color, quarter size or larger round cups. Inside the cup-like culprits is where the artillery blast comes from. You can see the empty cup where the shots were fired, or you may see the actual sticky mass that have not been fired yet.
REMOVING THE SPOTS
First off, very difficult if not impossible to remove without damaging the siding. You can go online to find products that may help, but good luck.