Blackwell was drafted in the 24th round by the Texas Rangers in the 2009 Free Agent Draft from Clear Creek High School. Began his career with a 11-3 record for the Arizona Rookie League Rangers, posting a 3.86 ERA with a 91-25 strikeouts-to-walks ratio in 109 2/3 innings, allowing 107 hits. Promoted to Class A Hickory in the South Atlantic League in 2012, striking out nine hitters in seven innings before being sent to Spokane in the Northwest League appearing in 18 games, including two starts, pitching 45 2/3 innings and allowing 42 hits while walking 13 and fanning 27 with a 3.55 ERA. Spent 2013 with the Washington Wild Things of the Frontier League, posting a 4-10 record with a 5.36 ERA. Was third on Wild Things in innings (100 2/3), starts (18) and hits allowed (99). Is attempting to improve his mechanics in the TWL, which he believes may have hampered his development in professional baseball.