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A large bluet, with adults ranging from 28-36 mm in total length and 17-22 mm in hindwing length. Males are blue (andromorphic), while females are polymorphic and range from light blue to yellow-green (Forbes 1991). Males have narrow humeral stripes and a wide median stripe on the thorax. Black abdominal banding is present in both sexes. In males, the first three black bands occurring on abdominal segments 3 to 5 are typically short and similar in length throughout, however, the black bands may increase in width towards the end of the abdomen in certain northern populations. These males may also possess a large black spot on abdominal segment 2 that touches the rear margin of the segment. Male abdominal segments 6, 7 and 10 are mostly black on the dorsal surface, while segments 8 and 9 are mostly blue. Males have a black subapical bar on the abdominal segment 2 (dorsal). In females, the abdomen is mostly black on the dorsal surface, with pale basal rings on segments 3 to 8. Abdominal segment 8 in females may have more blue or light brown than other segments, and at most may fill segment. When present within abdominal segment 8, the front margin of the black band typically tapers to a point on the dorsal surface. Female abdominal segments 9 and 10 are black. Male eyes are dorsally black and ventrally blue, while female eyes are dorsally dark brown and ventrally light brown. Postocular spots are large and may form dumbbell shape in both sexes. Boreal bluets are virtually indistinguishable from northern bluets (Enallagma annexum) except when viewed under magnification. Male boreal bluets and northern bluets differ in clasper morphology (McPeek 2011). Hagen’s bluets (Enallagma hageni) and marsh bluets (Enallagma ebrium) are similar in colouration, but both are notably smaller, may have smaller postocular spots, and have a larger spot on abdominal segment 2. Familiar bluets (Enallagma civile) are also similar in appearance, however the cerci are longer than paraprocts in familiar bluets, while the opposite is true for boreal bluets (Acorn 2004; Paulsen 2009).

As with most bluets, males congregate over open water in large numbers and will perch on emergent vegetation, alert to passing females (Paulson 2009). Gatherings of males may be so large that a bluish film becomes apparent over the water’s surface. Pairs form over or adjacent to water bodies, and may become very abundant in woodland corridors. Copulation lasts 10 – 27 minutes, averaging at 23 minutes (Paulson 2009). Males and females may break tandem for submerged oviposition, which, on average, lasts 23 minutes. Surface oviposition generally lasts longer, averaging at 46 minutes. Eggs hatch relatively quickly after oviposition and nymphs develop quickly, reaching late instars by October (Duffy and Liston 1985). Nymphs are typically ambush predators and feed on mobile prey (Koperski 1997). After overwintering in their aquatic stage and reaching their final instar in early spring, nymphs climb out of the water onto a favorable perch from late May to July (Duffy and Liston 1985; Acorn 2004). Here they undergo their final molt, and the adult winged form emerges (Acorn 2004). Reproductive adults live up to a maximum of 17 days, but average life expectancy is 4 days (Paulson 2009).

Adults feed on a variety of small soft-bodied winged insects (e.g., mosquitoes, mayflies, small moths) (Paulsen 2009). As with most members of Coenagrionidae, nymphs feed on a variety of mobile prey, primarily aquatic invertebrates (e.g., mosquito and other fly larvae, mayfly larvae) (Lung and Sommer 2001).

Boreal bluets are common throughout north western North America, from uplands of Mexico to southern Alaska, and extend through central and eastern North America to the Atlantic coast. Less common than the northern bluet along the Pacific coastline (Acorn 2004; Paulsen 2009).