Natural predators usually deal effectively with black fly on things like flowers as the season progresses, so although they are unsightly, don't worry, as the plants will outgrow them and the black fly are unlikely to affect the flowers. Encourage hover flies and ladybirds into your garden. There are also biological controls you can buy which will eat black fly.
Although personally I wouldn't do anything at this stage, it could be that the main problem why you can't get rid of them is because they are being protected against predators by ants, which climb up and down the stems and 'milk' the honey dew or sugary sap, from the black fly's bodies. If this is the case, then get rid of the ants first.
Try using soapy water as a spray as that is just as effective as most of the pesticides and far less harmful to other wildlife.

If your sunflower leaves are sort of pleated horizontally (crinkled) this is the result of damage of some kind - could be a sudden cold snap at the point where they were growing, or injury of some kind. Nothing to worry about and unrelated to aphid infestation.