Set a pizza stone on the bottom of the oven and preheat the oven to 450°. In a food processor, combine the black beans with the roasted red peppers, garlic clove and chili powder and puree until smooth. Season with salt.
Stretch the pizza dough to a 12-inch round and transfer to a cornmeal-dusted pizza peel. Spread the black-bean puree on top, leaving a 1/2-inch border. Sprinkle with the cheese, tomatoes, olives and scallion.
Slide the pizza onto the preheated stone and bake for 12 minutes, until the bottom is crisp and the cheese is bubbling. Transfer the pizza to a work surface and top with the lettuce and dollops of the sour cream. Cut into 4 or 6 wedges and serve at once.