“If anybody’s in distress, who do you call?” she asked. “White people will very rarely be that for a person of color unless that white person has done work in understanding racism.”

Absent deep, honest conversations about race with their black friends, white people can be left with superficial relationships and without a real understanding of how race plays out in a black person’s daily life. “Having black acquaintances and having black friends is different,” said Nick Adams, a television writer who is black and the author of the book “Making Friends With Black People.”

But sometimes it’s the relationships that white people have with black friends that can lead them astray. They can be lulled into a false sense of familiarity that might have them pushing boundaries better left untouched.

“Know the line,” Michael Harriot wrote in a semisatirical article in The Root that offered seven rules for white people with black friends. “While you might come from a long line of habitual line steppers, please know that there are some thresholds you cannot cross.”

Ms. DiAngelo said she once crossed that line, in a meeting with three black women, two of whom were close friends and another who had been hired as a consultant to design their website. Ms. DiAngelo told the consultant that her friend — one of the other women sitting right with them in the meeting — had a bad experience conducting racial justice training at a mostly white company. Feeling at ease, and eager to convey her understanding of how racism played out in that professional environment, Ms. DiAngelo joked that the company probably did not want her black colleague to return because “her hair must have scared the white people.”

It was only later, when one of her co-workers told her that the remark had offended the consultant that Ms. DiAngelo said she realized that her closeness with two of the black women in the meeting had lulled her into being too comfortable.

“I just kind of took for granted a relationship that I did not have,” she said.

The black friend as a buffer dates to the days of slave owners painting a false narrative that they were beloved by the African-Americans they subjugated. The slave owners “sought to justify how their racial hierarchy harmonized relationships between a dominant (white) race against a subordinate (black) race,” Tyler Parry, an associate professor in African-American studies at California State University, Fullerton, wrote in the Black Perspectives blog.