Females Osmia caerulescens are 10–11 mm in length, they are dark blue-black bees with a metallic sheen, which are sparsely covered with brown hairs with those on the abdomen forming a narrow, dense, flattened band on the hind margin of each of the segments. The dense brush of pollen collecting scopae on the underside of the abdomen is jet black. The males are slightly smaller, 9mm, more slender in build, distinctly metallic green and clothed with pale hairs.