Heat a dry griddle or heavy skillet over moderate heat until hot but not smoking and toast chilies, 1 or 2 at a time, pressing down with tongs, a few seconds one each side to make more pliable. Wearing rubber gloves, remove stems, seeds, and veins. In a blender puree chilies with remaining ingredients until completely smooth. Sauce may be made 2 days ahead and chilled, covered.
