Blackburn was optioned to Triple-A Las Vegas on Sunday, Jane Lee of MLB.com reports.
Blackburn threw the ball well early in camp, but he's been sent to Triple-A where he'll open the 2019 campaign. He's surfaced in the major leagues with the Athletics in each of the last two seasons yet struggled to a 7.16 ERA with 19 punchouts over 27.2 innings last year.