Overall length of 6".  Male has a black crown and nape with a blue-gray back.  Female has gray or dull black crown and nape with a gray back. 
 
    Distinguishing aspect of this bird is that it creeps headfirst in a downward direction along the trunks of trees as it looks insects.  Easily drawn to bird feeders that offer sunflower seed, safflower, and suet. 

Below are other webpages with more info on this bird:

Chipper Woods Bird Observatory

USGS Patuxent Bird Identification InfoCenter