Peel cucumber and cut in half lengthwise; scrape out seeds with a spoon and discard. Dice cucumber (about 1/4 in.) and put in a large bowl. Peel and chop onion; rinse and drain. Rinse and stem tomatoes and chile; chop tomato and mince chile. Add onion, tomatoes, chile, black-eyed peas, cilantro, 1 teaspoon salt, and pepper to cucumber. Mix well.
Up to 30 minutes before serving, mix in lemon juice. Add more salt to taste.