Dragon tongue beans are a variety of the species Phaseolus vulgaris, which includes wax beans, green beans, and yard long green bean. These beans look somewhat like green beans except they are creamy white with numerous streaks of bright purple on the pods, giving them the appearance of a dragon's tongue. The pods are four to five inches in length and are wider and flatter than green beans. Like other snap beans, both the pod and beans are edible.
Look for dragon tongue beans in specialty supermarkets or farmer’s markets. The best time to find them is during green bean season in summer. Select beans that are firm and unblemished. The beans will keep in the fridge for two to five days. When ready to cook, rinse the beans in water and snip off both ends with a knife or fingernails. The entire pod of the beans is edible and the beans can be steamed, boiled, or blanched like green beans. The pods can also be pickled. Be careful when cooking the beans, as the purple color will start to vanish when the beans are heated. Overcooked beans will have no purple streaks. Try to undercook the beans to retain some purple color. The pods taste like green beans but also are said to have a mild spicy flavor.
http://www.1webblvd.com/coosemans/prod001.htm (for a picture of the beans)