There’s a definite downward trajectory to Deon Taylor’s “Black and Blue.” It begins with a compelling sequence that speaks to the friction between African-Americans and the police in American cities today, suggesting a probing social commentary. By the time it concludes, though, it’s turned into a pretty standard-issue action movie without much of a brain in its head.

That opening shows Alex West (Naomie Harris) taking a run through a nice New Orleans neighborhood, only to be stopped by a police patrol car. Two white cops emerge and treat her roughly, tossing her against a fence. But when they look in her wallet, they discover she’s on the force too. They don’t apologize, merely noting that she fit a description, and are on their way.

So is Alex, to precinct headquarters, where she suits up for her tour with her partner Kevin (Reid Scott). During their shift they stop at a convenience, where Alex recognizes the clerk, Mouse (Tyrese Gibson), from when she lived in the projects—before she left for the army. In the parking lot she bumps into Mouse’s sister Missy (Nafessa Williams), who was once her best friend but is now with local drug kingpin Darius (Mike Colter) and, like the others in the old neighborhood, is of the opinion that Alex has gone over to the other side by putting on the blue uniform.

Back at the precinct, Alex volunteers to take Kevin’s night duty. That means partnering with burly Deacon (James Moses Black), who’s not happy being stuck with the rookie. And with good reason: he’s part of a bunch of crooked cops, headed by undercover narcotics detective Terry Malone (Frank Grillo) and his partner (Beau Knapp), who are in league with Darius but now under pressure from investigators looking into corruption in the department.

During their shift, Deacon makes a stop at a deserted factory where he says he’s meeting an informant. He tells Alex to say in the car, but when she hears gunshots she goes in, only to witness Malone shooting three unarmed young black men—something captured on her body-cam. Malone’s partner shoots her, but she manages to escape with the camera, and the chase is on. She’s pursued by all the cops who are part of Malone’s crooked crew, as well as Darius’ gang after Malone tells him it was she who shot the boys—one of whom was the drug lord’s nephew. Her only ally in the dangerous effort to get the truth out—by posting the body-cam footage—will be Mouse, though she also tries to enlist the frightened Kevin to help.

By this time, despite some sporadic allusions to the mutual suspicion between the African-American community and the police (even those who are black), the movie has degenerated into a formulaic urban action picture that grows increasingly silly with each successive plot twist. By the time that Alex faces off against Malone and Mouse worms his way into the police station, the movie has long passed the point of plausibility.

Nonetheless the movie has been competently made, with Taylor handling the action reasonably well despite some muddiness in the more hectic sequences toward the close (Dante Spinotti was cinematographer and Peck Prior edited). Harris and Gibson give intense, committed performances, and Grillo adds his patented wacko sneer to the proceedings—he’s become one of today’s go-to actors for amoral, brutal thugs, on either side of the law. Of the others Colter and Knapp add to the ensemble of snarling villains, while Scott and Black provide a modest dose of complexity as officers who fall into the grey area rather than the black-or-white one, if you’ll pardon the expression.

So long as one is willing to settle for a conventionally plotted urban action movie, “Black and Blue” will suffice, but its fleeting bits of social commentary suggest it might have been something more.