Blackish Nightjar is crepuscular and nocturnal. These nightjars spend the day roosting, often on rocks or fallen trees. Activity begins at twilight around 18:30 and birds return to roost sites around 05:30. Birds forage in the air over rocky outcroppings and above the tree canopy. Blackish Nightjars have also been seen running across large rocks, picking prey from the rock’s surface (Cleere and Ingels 2004). They sing from low perches in bushes, small trees or from ground in the twilight or moonlight (Cleere 1998, Jackson and Ingels 2010).
Blackish Nightjars are reported to be "not shy" and to respond aggressively to song playback (Jackson and Ingels 2010).
When chicks are approached too closely by humans they respond with a peep; sometimes this results in the appearance of a parent who returns alarm calls and the performance of a distraction display. Adults walk away from the nest, bobbing up and down and dragging their wings behind them in attempt to lure visitors away from the nest (Cleere and Ingels 2004, Solano-Ugalde et al. 2012); meanwhile the chick seeks cover in nearest vegetation (Cleere and Ingels 2002). Males have also been observed roosting during day with older young (Cleere and Ingels 2004).
There are no published data on territorial defense, maintenance, or home range size for Blackish Nightjar. Blackish Nightjars respond aggressively to song playback (Jackson and Ingels 2010), but sometimes nests semi-colonially. The dynamics and territoriality of this nesting pattern have not been established.
Single males displays to single females on fallen trees or sandy patches by angling their tail towards the female to show off white markings. Females have been noted to occasionally gape widely. Throughout this display male and female blackish nightjars communicate with pru-r-r-r-t calls (Cleere and Ingels 2004).
No information currently available about whether this species, like many nightjars, is monogamous.
Social and interspecific behavior
Blackish Nightjars usually roost in pairs side by side. They may occur semi-colonially on large outcrops during breeding seasons however other sightings report single nests (i.e. not all individuals semi-colonial) (Jackson and Ingels 2010). A male and fully-grown young were reported roosting close together on dead tree near the Manu River. The male foraged continuously above the river, but always returned to roost. The young made few foraging flights, but the male was also observed feeding it while perched on a log. Another male was reported to a shade three week old individual when roosting in full sun (Cleere and Ingels 2004).
The eggs of the Blackish Nightjar are reported to be predated by reptiles, birds and mammals,but no observations of predation are reported (Ingels et al. 1984). No information on adult predation available.