Those are black aphids. Conventional insecticides like Bayer Complete Insect Killer, famous blue bottle, at 1 T./gallon of water, a couple sprays a week apart, will kick them to the curb. Organic methods don't work effectively for me, so someone else would have to chime in on this attack method.

Left to multiply and develop colonies, under leaves and at growing tips, aphids of any color can do a lot of damage.

The amount of force needed to spray them off with water which is often recommended could damage leaves and tender new growth aphids frequently infest.

Moses