he!  face_off's real skin shader and other skin shader products make pretty great freckles with procedures.  i'd have to double check, but i think it uses the spots nodes for the generation of the basic distribution.  just a mention, you can use a lot more than a blender node to mix.  you can use math, color math, and color ramp as well.  and different combinations of nodes for greater blending options.  color ramp might actually be useful if you want differently colored freckles as in the above picture.