"I should have never let him go the second time," Towers said Monday afternoon from Orlando, Fla., where he is attending baseball's annual General Managers Meetings.
It makes no difference now, as the two will be reunited in Arizona.
Blum, 37, was signed to provide manager Kirk Gibson with flexibility off the bench and as a player who can pinch-hit, bounce to several positions in the infield and even play the outfield in a pinch.
"His intangibles ... he adds experience to our bench; he can play about every position," Towers said. "He's a weapon late in the game and will help Kirk out a lot."
Towers has seen firsthand how Blum -- who played first, second, third and shortstop in 2010 and has also played left and right field in his career -- can help a team.
Blum, who hit .267 for the Astros last year in his 12th season in the big leagues, signed with the Padres as a free agent before the 2005 season. He was traded to the White Sox on July 31 of that season before signing with the Padres again that November.
In 2006, Blum stepped in and played 49 games at shortstop after Khalil Greene suffered a torn middle finger tendon. That helped keep the Padres on top in the National League West, a division they went on to win.
Blum declared free agency after the 2006 season only to sign with the Padres for the 2007 season, his last in San Diego. Towers said that he wanted to re-sign Blum for 2008 but the Astros made him an offer (one year, $1.1 million) the Padres couldn't afford.
Blum then signed with Houston, where he played the last three seasons. The Astros declined to pick up Blum's $1.65 million mutual option for 2011.
Towers has stressed since taking over the reigns in September his preference for having veteran hitters coming off the bench.
"It's hard for young guys to be effective pinch-hitters coming off the bench," Towers said in late September. "If you don't have that experience or those guys that are threats, it makes it pretty easy on the opposing manager, because he has no fear of anyone that is coming off the bench."
The D-backs had young players like Rusty Ryal, Gerardo Parra and Brandon Allen coming off the bench at times last year. Towers believes Blum can help mentor these young players.
"He's a team guy and a winner," Towers said. "We need more of those guys, especially with a younger team."