Ayocote negro, or black runner beans, are one of many rare and unusual "heirloom beans" you can buy in the stalls behind Canino's. They aren't cheap -- I paid something like $5 a pound. I also saw mayocoba beans, cranberry beans, and black and white vaquero beans there.
But I couldn't resist the ayocote negro. These gorgeous, shiny black beans were very popular in Mesoamerica before the Conquest, according to heirloom bean expert Steve Sando. Sando says that few Mexicans cook them anymore.
Sando's company, Rancho Gordo, has launched a project to revive heirloom beans and help Mexican farmers by paying small farms in Mexico to grow the seldom seen varieties for the American market. The idea is to encourage biodiversity, save small farms in Mexico, and end up with a whole lot of interesting beans. Sando's website has recipes, he also has an excellent cooking blog dedicated to New World food traditions.