Clubs pay $50,000 to select a player in the Rule 5 Draft, to be held on Dec. 6 at the conclusion of the Winter Meetings in Nashville. If that player doesn't stay on the 25-man roster for the full season, he must be offered back to his former team for $25,000.
With the additions, the Cardinals' 40-man roster is at 39 players.
Blazek, ranked by MLB.com as the Cardinals' No. 13 prospect, spent the majority of the 2012 season at Springfield, where he posted a 4.16 ERA in 40 appearances (seven starts). He also made two appearances with Triple-A Memphis, including one start.
A 35th-round selection in the 2007 First-Year Player Draft, Blazek transitioned into a relief role this season after pitching primarily as a starter in previous years.
Fornataro, ranked No. 17, moved into a bullpen role in 2012. That helped boost his velocity into the upper-90s.
Fornataro, 24, made 57 appearances and posted a 2.39 ERA with Springfield. In addition to recording five saves, Fornataro collected 41 strikeouts in 67 2/3 innings. He was particularly tough against right-handed batters.
Butler, 23, led Springfield with 25 saves, finishing second in the Texas League, and he was 5-1 with a 2.76 ERA in 53 appearances -- all in relief. He was the Cardinals' 24th-round selection in the 2009 Draft.
Siegrist just wrapped up a stint in the Arizona Fall League, where he went 2-1 with a 2.37 ERA in six games (five starts) for the Surprise Saguaros. The lefty struck out 27 and walked just six in 19 innings. The time in Arizona helped Siegrist make up innings that he missed during the regular season due to time on the disabled list with a shoulder strain.
The 23-year-old Siegrist split the season between Class A-Advanced and Double-A and finished with a combined 2.77 ERA.
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.