Cut tomatoes into fourths. Place in small saucepan with just enough water to cover the tomatoes.
Add salt and bring to boil. Simmer tomatoes for about 20 minutes.
Wash chile de arbol with cold water. Pat dry and remove stems.
Toast the chile de arbol in a toaster oven on aluminum foil. Watch closely, as chile can burn quickly. Toast until dark brown. If you don't have a toaster oven, toast in a dry pan, turning chile often so it doesn't burn.
Place toasted chile in the blender and blend until it is finely powdered. Place a paper towel under the lid of the blender to keep powder from escaping.
Add tomatoes and 1/4 cup of the liquid to the chile in blender and puree until smooth.