Blackley, outrighted by the Astros, has been assigned to Triple-A Round Rock. He will not be on the Rangers' 40-man roster.
Rangers general manager Jon Daniels said Tuesday the club would look for protection for its big league pitching staff since starters Matt Harrison and Colby Lewis were lost for the season in the past week.
Team doctors decided Tuesday that Harrison, who had a pair of back surgeries in April and May, needed rest after three rehab starts caused elbow inflammation. Harrison was overcompensating because of the back issue, Daniels said. The 27-year-old Harrison is expected to be ready for spring training in 2014.
Lewis, 34, will have hip surgery next week in New York City. The Rangers are open to re-signing Lewis, who is a free agent at the end of the season.
Blackley, 30, has spent the entire season with the Astros organization. He is 1-1 with a 4.89 ERA for 42 relief appearances for Houston, allowing 10 home runs.
Blackley has eight career wins, four of them against the Rangers. The Australian native has held left-handed batters to a .231 average for his career.