A longtime friend of almost 30 years, the A-lister admitted he and Jean Black are like “family,” and their bond grew even stronger after an embarrassing incident in the early ’90s.

When working on 1994’s “Legends of the Fall,” Black had to use cosmetics on Pitt’s butt to even out a tan line. “When it comes up, we can’t really look each other in the eye,” Pitt recalls.

The 56-year-old’s cheeky confession came during Drew and Jonathan Scott’s latest HGTV show, “Celebrity IOU,” where the homey twins help stars give back to the non-famous people in their lives with a surprise home renovation.

Pitt wanted to give back to Black, who works with other high-end clients like Julia Roberts and Josh Brolin — although Pitt claims he is her favorite.

“She’s family, we’re like brother and sister,” the actor says in his episode, which airs April 13. “She’s been that person I value so much in my life.”

Together, they’ve done over 40 films, including “Once Upon a Time In . . . Hollywood,” which he won an Oscar for this year.

When Black went away for three weeks, Pitt and the Scott brothers got to work remodeling her Santa Monica, Calif., stand-alone garage into a guest suite with a bedroom, kitchen and bathroom, makeup studio and storage unit.

Pitt, who wore his standard long-sleeve shirt layered under a short-sleeve tee and a scally cap every single day of filming, did not shy away from getting his hands dirty.

“I love the sound of a construction site. If I’m not building, I’m dying,” he says. “I just walk into a place and just see the possibilities.” He was quick to pick up a hammer and join Drew and Jonathan in demolition. (And no, Brad Pitt can’t tell Drew and Jonathan Scott apart, either.)

The father of six also didn’t shy away from flexing his design savvy. “She’s got a lot of these washed-out pewters and taupes in her place,” says Pitt, while picking out flooring. “So that would be a natural carry-through.”

“I’m extremely tactile. In fact, I prefer the design be more in the materiality than actual decor or decoration. Seeing how the materials relate to each other and the feeling you get living amongst them,” he monologues.

Pitt was enamored at the final result. “It was such a s - - t box, this is amazing. It’s fantastic,” he says. “I’ve been waiting so long to see something happen to this dump.”

When the three men surprised Black, she was equally shocked. “I’m really just so flabbergasted,” she says. “It’s really very moving and I just so appreciate it.”

“I know Brad’s generosity, it’s huge,” she continues. “But for him to do this, it’s really more than I ever thought could happen. I am so touched by this, I could really not thank you enough.”

“Celebrity IOU” premieres on HGTV at 9 p.m. on April 13.