Rinse the chiles in cold water and remove the stems. Cook in the boiling water until tender, about 15 minutes.
Remove the chiles, reserving the liquid.
Place a few of the chiles in a blender or food processor with 1/2 cup of the reserved liquid, and blend to a paste.
Remove to a bowl. Repeat with the rest of the chiles.
Heat the oil in a skillet. Add the garlic puree and flour, stirring until the flour browns. Add the chile paste, stirring continuously until it all boils and thickens. Season with the salt. Thin with the cooking liquid.