Answer: It's difficult to identify an insect without really seeing it, but my experience has been that nasturtiums are magnets for black bean aphids. So, without even seeing the plant I can agree with your opinion - black aphids are covering your plants. Insecticidal soaps work well against soft bodied insects such as aphids, but they do not harm beneficial insects. Safer's, as well as other manufacturers, have insecticidal soap products available in ready to spray containers. Repeat applications will be necessary. Be sure to spray the undersides of the leaves and the stems of the plants.
Best wishes with your garden!
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