Blue dasher males and females look quite different. Both have a white face, a black abdomen tip, and slanted black and yellow stripes on the thorax. There is an amber cast on the hindwing bases. But males are blue and females are striped black and yellow.
Mature blue dasher males are chalky blue with a dark abdomen tip, white face, and turquoise eyes. There's a wedge of rust or yellowish color on each side of the front part of the abdomen, behind the wings. Younger males look like females. Males intermediate in age display a mix of mature male and female characteristics.
Female blue dashers have a pair of yellow stripes on each side of the top of the abdomen; a white face; and (usually) eyes that are divided into two sections: rusty brown above and sky blue or gray below. There is a small tuft of white or pale yellow at the tip of the abdomen.
Learn more about this and other dragonflies on their group page.