The "Five Easy Pieces" star was 74.
"She was a stellar person in every area of her life," Black's agent Sarabeth Schedeen wrote. "Smart funny talented tenacious supportive and loving. Everyone who knew her will miss her."
Earlier this year, Black began a crowd-funding website where she raised thousands of dollars that would help pay for treatments she hoped to get.
He husband Stephen Eckelberry posted a message for donors Wednesday explaining that the actress' health had continued to deteriorate at an "alarming pace."
"She became bed-bound: the spreading cancer having eaten away part of a vertebra and nerves in her lower back," Eckelberry wrote. "Her left leg stopped functioning. We could not go to Europe as we had hoped. It would have been almost impossible to travel to the airport. So we brought alternative treatments to her bedside."
Eckelberry, who captured his wife's struggles on film over the past three years, said that she survived weeks longer than doctors predicted and that he plans to release the the footage at one point "on the right platform."
"I hadn't planned on doing anything with the footage, until a few weeks ago, Karen reached out to her old friend, Elliot Mintz," he wrote. "Elliot is considered a media guru who has offered advice to dozens of famous clients over the years including Karen."
Black is best known for her work in "You're a Big Boy Now," "Easy Rider," "Family Plot" and "Five Easy Pieces," for which she was nominated for an Academy Award.