Blunt posted his announcement on YouTube just minutes after he had informed Republican leaders of his decision in a telephone conference call just before 4 p.m.
The decision came as a shock to most legislators who said they had no prior knowledge of his decision.
"It was a big surprise," said Lt. Gov. Peter Kinder, who echoed the reaction from other Republican leaders who said they had absolutely no hint of the governor's plans.
Democrats also expressed surprise.
"I certainly think it's the surprise of session so far," said Sen.Chuck Graham, D-Columbia.
Attorney General Jay Nixon, seeking the Democratic nomination for governor, issued a short statement wishing "Gov. Blunt and his family all the best in the future."
Blunt's announcement suggested family considerations were a factor in his decision.
"I have spent more time away from them than I would like," Blunt said in the video statement with his wife at his side.
Blunt also suggested he had lost some of the passion for politics.
"Because I feel we have changed what I wanted to change in the first term, there is not the same sense of mission for a second," he said.
Republicans who listened to the conference call from the governor said he also expressed a loss of desire for a political campaign.
"What he told us was that he just did not have the same sense of mission and purpose," said Senate President Pro Tem Mike Gibbons, R-St. Louis County. "Resolute and at peace were the two hallmarks of what he had to say to us."
Kinder said Blunt had told them he "had prayed a lot about it over the last 10 days and arrived at this conclusion."
A better sense of the governor's thinking may come at the 9:30 a.m. news conference the governor has scheduled to discuss his decision.
Graham gave a more political possibility for Blunt's decision. He said Blunt's previous term did not present a viable chance for re-election.
"Considering his record of slashing health care for the elderly, poor and the disabled in this state, considering the polling numbers that had him down consistently in double digits to Jay Nixon over the last nine months, when you look at it from that perspective, I guess it shouldn't surprise us because it was going to be a very tough re-election campaign," Graham said.
Many Republicans, however, defended Blunt's record and boasted of his success.
"The governor has given great leadership to the state of Missouri," Kinder said. "He said that his sense of mission is basically accomplished, that what he promised the people he would do in '04 he has done and delivered on."
Other legislators cited the stresses of a re-election campaign as a possible factor.
"For somebody that's going into a re-election campaign, it's no fun looking to the next year being gone every night much less have a little baby that you want to be away from every night, seven days a week, and you got to do that to campaign for governor," said Sen. Kevin Engler, R-Farmington, who said he had talked to the governor after the announcement.
Blunt's action caused a flurry of speculation about possible Republican replacements. Kinder issued a short statement suggesting he might switch to a gubernatorial campaign. Other Republican names mentioned include state Treasurer Sarah Steelman, U.S. Rep. Kenny Hulshof and former U.S. Sen. Jim Talent.
As Republicans were looking for a replacement, Democrats were sounding upbeat.
"As a Democrat, certainly I am excited about the wonderful opportunity this brings forth for our next Democratic governor, Jay Nixon," said Senate Democratic leader Maida Coleman, D-St. Louis. "I'm looking forward to a campaign that will only become even more exciting as we get closer to the election."