William Grier. (Geoffrey Grier via AP)

The film was shot almost 50 years ago, but the fraught conversation about race and racism it captured could have been held on the streets of Ferguson, Mo., or Baltimore yesterday.

On a late summer day in 1968, two black psychiatrists took a walk with a white reporter on a grim-looking block of San Francisco. As cameras rolled and bystanders waved, the men tried to explain why the racial unrest sweeping America — seen most tragically in the assassination of Martin Luther King Jr. months before — wasn’t just the result of political groups like the Black Panthers or teenagers behaving badly.

“The decision-makers … operate sometimes from not a very sophisticated point of view if, for example, they feel that riots occur because there are too many youngsters on the block in August,”  said William Grier — keeping pace with Price M. Cobbs, with whom he co-authored “Black Rage: Two Black Psychiatrists Reveal the Full Dimensions of the Inner Conflicts and the Desperation of Black Life in the United States.” “What we’ve tried to do … is give them a deeper understanding of why there is a racial crisis in this country. That is not a superficial issue. And that it is some 300 years in length.”

Grier died last week at 84 in hospice care in California after battling a brain lesion. But the legacy he and Cobbs built with “Black Rage” is perhaps more relevant than ever.

“You can try to come up with another name, call it ‘Black Lives Matter,’” Gregory Grier, Grier’s eldest son, told the Associated Press. “Whatever you want to say, at the end of day, there is black rage. The relevance of what they were saying is really, really on point now.”

[How Black Lives Matter moved from a hashtag to a real political force]

Grier was born in 1926 in Birmingham, Ala. — the heart of the Jim Crow South. As Corrie Ort, his wife, told the AP, Grier’s father was fired from the post office for race-related reasons, so the family relocated to Detroit. Educated at Howard University and the University of Michigan, he ended up practicing psychiatry in San Francisco in the 1960s.

There, after a divorce, he was running a children’s psychiatric clinic when he met Cobbs — one of just a few black psychiatrists in the city.

“Bill had an edge,” Cobbs — more frequently in the media than Price — wrote in his memoir in 2006. “He was direct. He didn’t suffer fools. He was a man who understood clearly the psychological burden of a people who had been pushed to the outside, forced to stay there, and given very little room for moving from that exiled state of being.”

The idea behind “Black Rage” was simple. As shown by phrases such as “the Negro problem,” African Americans were almost considered an undifferentiated mass at the time. Why not ask them about their struggles in the same way a psychiatrist would analyze a patient?

“We began to talk about our patients, and we realized that anger was bubbling up,” Cobbs told NPR in 2006. “We then began to put together a systematic study of our patients, a systematic study of the literature, and incidentally, we talked about the anger within ourselves. And we were then able, through a very big study of the black experience at that time, to put our finger on, oh, this anger is going to erupt and erupt it did.”

The results were not encouraging.

“All blacks are angry,” Grier and Cobbs wrote. “White Americans seem not to recognize it. They seem to think that all the trouble is caused by only a few ‘extremists.’ They ought to know better. We have talked to many Negroes under the most intimate of circumstances and we know better.”

By way of introduction, the doctors discussed the cases of African Americans “not in open rebellion against anyone,” including a painter and the daughter of a land-owning “Negro farm family.” Both, they found, struggled not just because of unique personal circumstance, but because of their race.

“They are typical of black Americans,” they wrote, “held tight in a snare, coming more and more to realize that even their inner suffering is due largely to a hostile white majority and, with this realization, gaining a determination to change that hostile society.”

Black Rage” did not have a happy ending.

“The tone of the preceding chapters has been mournful, painful, desolate, as we have described the psychological consequences of white oppression of blacks,” Grier and Cobbs wrote. “The centuries of senseless cruelty and the permeation of the black man’s character, with the conviction of his own hatefulness and inferiority, tell a sorry tale. This dismal tone has been deliberate.”

Published the year King was gunned down and parts of the nation rioted, “Black Rage” was a phenomenon. In 1969, ABC produced a show about the book.

“There had been nothing like it before, and many of the reviews agreed,” Cobbs wrote in his memoir. “… We felt we had written with an authentic black voice that had authority and credibility. A great deal had been written about similar issues in the black experience. But little of it had the direct immediacy that fuels ‘Black Rage.’”

Not everyone agreed. Writing in the New York Times, African American psychologist Kenneth Clark offered a takedown of the book, criticizing its “generalizations,” anonymous case histories and focus on sex.

“The authors of ‘Black Rage’ have joined the present fashionable cult of literate black and white flagellants who now believe that America’s racial problem can be clarified and racial justice obtained through a sadomasochistic orgy of black rage and white guilt,” Clark wrote.

In a stunning letter to the editor that reads more like a manifesto, Grier and Cobbs — dismayed that “after 62 highly favorable reviews, the 63rd and first critical comment came from a black brother” — responded.

“We wrote a book designed to involve the reader,” they wrote. “We deliberately did not footnote our book, knowing that no social revolution is brought about by footnotes. ‘Black Rage’ is an expression of advocacy psychiatry designed to stir the reader to change.’

Grier and Cobbs signed the letter “yours in the bondage of brotherhood.”

The pair wrote another book — “The Jesus Bag,” in 1971 — that took on the role of Christianity in the black community. That book too was criticized for its methodology, and did not prove as popular.

“As a black person, I respect Cobbs and Grier for embarking on what is, for the most part, an uncharted course,” Douglas Johnson wrote in The Washington Post. “However, we ought to be able to demand of them a more scientific indication of what they, as profession explorers of the mind, see as the outlines of human psychology that are especially black.”

Grier was also the father of comedian David Alan Grier, who wasn’t above distancing himself from “Black Rage” when convenient.

“I remember in high school I had this white English teacher who kept ribbing me to write a paper for him,” Grier told The Post in 2000. “My response was ‘[Bleep] you, man, I’m not going to sell out.’ But my teacher said, ‘I promise if you write me a real book report, I’ll give you an A.’ So, of course, I ripped my father’s book to shreds.”

Still, from King to Ferguson and beyond, “Black Rage” — and rage about black rage — remains on front pages everyday. And what was once thought militant is now, and perhaps always has been, mainstream.

“The militants don’t produce white hostility, they simply expose that which is already there,” Grier said in 1968.

Correction: An earlier version of this article referred to Kenneth Clark as a “psychiatrist.” He was a psychologist.