Barton was the longest-tenured player on the A's. The AL West champions needed the roster space after claiming right-hander Danny Otero off waivers from the New York Yankees and righty Josh Stinson off waivers from Milwaukee.
"With the kind of depth we have, we're going to make some tough moves here," AL Manager of the Year Bob Melvin said.
Melvin said "you hope they land in a good place," but if not the team would welcome back Barton and Blackley in the big leagues later this season if they aren't claimed and go to Triple-A.
Otero was optioned to Triple-A Sacramento and Stinson was optioned to Double-A Midland. The A's also optioned left-hander Jordan Norberto to Sacramento.
"All of these are tough," Melvin said before Friday's exhibition game against the San Francisco Giants. "When you come to the end and you have as much depth as we do, you start with 25 and it doesn't mean in Norberto's case he won't be a significant a contributor as he was last year. That was a tough decision to make but it's one we had to make at this point. As far as the designation, we needed a couple roster spots for the two guys we picked up."
Blackley went 1-1 with a 14.21 ERA in seven games, two starts, this spring. He allowed 27 hits in 12 2-3 innings. He was claimed off waivers from San Francisco on May 15 last year and went 6-4 with a 3.
"I kind of saw it coming," Blackley said at his locker in the visiting clubhouse at AT&T Park. "This is a great team and it's hard to crack it with a 14 ERA."
Blackley posted on his Twitter account a message and link to an in-depth thank-you note to the franchise and all its supporters.
"As this chapter ends, hopefully a new one begins," he said. "Thank you so much to Oakland and its fans!"
The 27-year-old Barton was batting .206 with five RBIs in 17 exhibition games this spring. He was acquired with two other players from the Cardinals in the December 2004 trade that sent Mark Mulder to St. Louis.
Barton batted .204 in 46 games last year with the A's and .249 with 27 home runs, 163 RBIs and 272 walks in 484 games in his Oakland career.
Also Friday, Melvin said he expects new shortstop Hiroyuki Nakajima will begin the season on the 15-day disabled list after he strained his left hamstring Tuesday.
"We'll know for sure Sunday, but it's very likely that he'll be on the DL," Melvin said.