The blue mud dauber, or Chalybion californicum, is a metallic blue species of the mud dauber wasp. The wasp has a slender body measuring about 0.5 to 0.75 inch long with a thread-like waist. Blue mud daubers are solitary and do not live in colonies.
Nests are commonly found in attics, barns, eaves of homes, garages and walls of various structures. The species does not build nests of its own, but instead takes the abandoned ones of the black-and-yellow mud dauber, using water to soften and re-mold nests. The species feeds primarily on the black widow spider and is rarely aggressive towards humans.