Blue Mud Dauber Wasp

The blue mud dauber (Chalybion californicum) is a metallic blue species of mud dauber wasp that preys primarily on black widow spiders. It does not build a nest, but uses nests abandoned by other mud dauber wasps. Like other mud daubers, it is rarely aggressive. It is similar in shape and color to the steel-blue cricket hunter (Chlorion aerarium). Also the male population does not have a stinger, therefore it cannot attack and its sole purpose is for reproduction.
Source: Wikipedia

Blue Mud Dauber Wasp