Blue mason bees (Osmia) are among the first bees of spring. Relatively large (honeybee-size), relatively slow, and BLUE METALLIC (most). Many look dark until they catch the light.
They are attracted to bee boxes with reeds, paper straws, or drilled holes for building their nests. The main purpose of a bee box is to see them in action (in my opinion). We're not really saving the bees with these things, but we sure can get to know them.
If you an amateur bee photographer, Osmia are simply "flashes of blue."
Last pic = Kathlyn Collins