Belongs to a group of medium-sized odonates characterized by pronounced sexual dimorphism in their adult stage. Mature males display a predominantly bluish-black or charcoal-colored body, which is the result of intense pruinosity developing over the thorax and abdomen as they age. They exhibit a total body length of approximately 30 to 35 millimeters and a wingspan of about 50 to 55 millimeters. Their wings are mostly hyaline and transparent, featuring a characteristic small dark brown or blackish patch at the very base of the hindwings. Conversely, females and immature individuals display a much more cryptic coloration, dominated by yellowish and brown tones with distinct dark longitudinal bands running along the dorsum of the abdomen. Their large, globose compound eyes are dark brown in females and turn almost completely black in fully mature males of Erythrodiplax nigricans.

Geographical distribution: Occurs primarily in the temperate and subtropical zones of South America. Its geographic range is well documented across central and northern Argentina, including provinces such as Buenos Aires, Córdoba, and Santa Fe. Additionally, its distribution extends continuously into southern Brazil (specifically in states like Rio Grande do Sul and Paraná), throughout Uruguay, and into several wetland areas of Paraguay.

Habitat: Prefers lentic freshwater environments such as permanent lagoons, marshes, temporary pools, swamps, and slow-flowing channels. This species is highly dependent on bodies of water with dense emergent and floating vegetation, such as reeds and cattails, which provide essential perching sites for adults as well as crucial shelter and foraging grounds for the aquatic larval stages.

Feeding: Exhibits strictly predatory habits throughout all phases of its life cycle. In the aquatic nymph stage, it feeds voraciously on a wide variety of small freshwater organisms, including mosquito larvae, microcrustaceans, and other small aquatic invertebrates. As flying adults, they become highly agile aerial predators, capturing their prey on the wing. Their diet consists of dipterans, small moths, beetles, and other soft-bodied flying insects, playing an important ecological role in controlling pest populations within riparian ecosystems.

Behavior: Displays highly territorial and heliophilic behavior, with peak activity occurring during the warmest hours of full sunlight. Males strategically perch on emergent vegetation or bare twigs near the water´s edge, using these vantage points to monitor their territory, intercept incoming females, or aggressively chase away rival males. During periods of extreme heat, they frequently adopt the "obelisk posture," pointing the abdomen vertically upward to minimize the body surface area exposed to direct solar radiation, thereby regulating their internal body temperature.

Reproduction: Takes place in or near water and begins with a rapid aerial courtship. Once the male grasps the female behind her head using his anal appendages, they form a copulatory wheel. Following fertilization, the female performs exophytic oviposition by flying low over the water surface and repeatedly tapping her abdomen against the water to deposit the eggs near aquatic vegetation. During this process, the male typically hovers closely nearby, performing non-contact guarding to protect her from competing males. The hatched nymphs undergo several aquatic instars before climbing out of the water to complete their final metamorphosis.

Conservation status: Is not currently evaluated on a global scale by the IUCN Red List; however, at the regional level, it is generally considered a species of Least Concern (LC) due to its wide distribution and stable populations. Nevertheless, localized populations face potential threats from wetland degradation, chemical water pollution, and habitat loss caused by agricultural expansion and urban development.