How can I get rid of aphids all over my purple hull peas?

Spray them down with horticultural oil

Rubbing or spraying the plants with onion water can help. Place some onion slices/pieces in a bowl with water and put the bowl in the sun, if possible. Leave it for a day or two and you will have a very smelly, yet effective solution. (I use containers with lids to contain the smell; however, the onion water can be quite agressive to plastic, so do not use anything that you will want to keep or use for other purposes afterwards).

Get some ladybugs. You can order them online.

My garden is covered in them...and aphids. They are only on my purple hull peas. Black eye peas right next to them are unaffected.

Horticultural oil will help!