Blackburn (finger) threw 40 pitches in Monday's side session, and will head to Low-A Stockton to begin a rehab assignment shortly, Martin Gallegos of MLB.com reports.
The right-hander threw two "double up" sessions while throwing 20 in each. Blackburn is currently on the injured list with a right middle fingernail avulsion. It's possible that he will need to make multiple rehab starts because he hasn't been on the mound since early in March, but Blackburn should be able to rejoin the Oakland rotation by the end of April if there are no setbacks.