A picture of the insect you see would help, but I know that can be very difficult. 

I think you are seeing white flies. White flies show up during warm, humid weather and in greenhouses. Plants that are densely planted can also increase these conditions. Their feeding weakens plants and they excrete a sticky substance called honeydew. To top it off, black sooty mold grows on the honey dew creating the blackened leaves you see.

You can wash down plants with a strong spray of water - get under the leaves - followed by spraying insecticidal soap. Don't mix your own, but purchase a ready-to-spray product. Spray during the cooler time of day as the soap can burn leaves if you spray during the heat of the day. Avoid spraying any bees.

You may want to thin out your plants a bit so you can get better air flow, better light to the plant stems (equals better bloom) and you can access the whole plant when assessing / scouting / treating. This is an all-around best practice for managing pests and disease on plants.

For more info, here's a good webpage on white flies: https://www.growveg.com/pests/us-and-canada/whitefly/