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It looks like an infestation of aphids, they congregate on the underside of leaves and are usually present in multiple life stages and colors.  Often they will attract ants which milk the aphids for the honeydew they produce.  It is hard to tell from the picture, but it is possible the black "dots" are young Chrysanthemum lace bugs, or just black aphids.  Fortunately the treatment is the same.

It is best to remove the most effected leaves.  Both insecticidal soap and horticultural oil can help with the rest of the plant.  They are both sprays that must contact the insect to work so you will need to spray the underside of the leaf and perhaps spray a day or two later to catch any new generations.  Do not apply 45hrs before a freeze.  To avoid overspray that can contact pollinators, it helps to use the products in early morning or evening when they are less active.  The best advice is to always follow the instructions with the product.

To prevent future infestations, it helps to remove the leaves and plants this fall as aphids often lay eggs at the base of the plant(s) they infect.

Here is some additional info on aphids that may be helpful: 

1402 – Aphids – PlantTalk Colorado (colostate.edu)

Good luck, if you have further questions, please let us know.

Sandra, Janet
Colorado Master Gardener
Boulder Extension