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I think you are correct in saying black aphids. Best thing to do for an outdoor grow is to blast em off with a water jet hose on an overcast day or in the shade then spray the plant down with an long lasting insecticide. Given its not in bloom anything works. Soap and water has killed em off before for me; as well as a week solution of chemical insecticide. The blasting them off part is best to get rid of most of them. They don't like to travel far so if you do it in an area the plants are not, its easy to leave them there to die. If your plants are in the soil and can't be moved also spray down the soil aorund the plant with soapy water. It won't hurt the plant and should take care of most of the aphids that fall off. Remember to plant some marigolds and other things that attract ladybirds to come and eat the aphids. Google says these plants are good as well, "cilantro, dill, fennel, caraway, yarrow, tansy, angelica, scented geraniums, coreopsis, and cosmos" as they eat the pollen in the flowers. I grow cilantro every year for the hoverflys it brings. They are also a predator for pests. Some plants can also be used as bait crops, this mean the pests like that plant more then your other ones and its grown only to be a "sacrificial lamb" to the slaughter of these pests. This link describes aphids being attracted to asters and sunflowers a lot. https://hort.extension.wisc.edu/articles/aphids-2/ Good Luck!