The victory gave the Patriots a first-round bye and the Gillette Stadium field an extra week to dry out.
Blount rushed for a career-high 189 yards, returned two kickoffs for 145 and set a club record with 334 all-purpose yards.
The Patriots (12-4) obtained Blount in an offseason trade with Tampa Bay and he shared playing time with other backs since then. On Sunday, the 250-pounder led the charge.
He also scored on a 36-yard run and set up a touchdown and field goal with kickoff returns of 83 and 62 yards.
"Every time I play I try my best to come out with the mindset to make a contribution to any part of the game, whether it's kick returns or running the ball or catching it," Blount said.
After his second touchdown, he hugged Belichick, who had a towel draped over his shoulders.
Tom Brady was content to hand the ball off rather than throw through rain drops against a team leading the NFL in sacks and second in interceptions.
"The water on the field at the end, it was just puddles," he said.
New England won its 13th straight home game against Buffalo, including 12-0 since Gillette opened in 2002.
The Patriots have the second seed and play in two weeks at home, where they're 8-0. Denver, which beat Oakland, has the top seed. New England began its game with a chance to finish in any of the AFC's top four playoff spots.
"We put together a great season, but it doesn't matter much now," Brady said.
After each of those touchdowns Blount had his long kickoff returns.
"Of course the ball is wet," Buffalo wide receiver Robert Woods said, "but we still have to make plays and catch the ball."
Before the Patriots ran the next play, "Singin' in the Rain" blared over the public address system, and drenched fans sang along.
The first two rounds of the NFL's postseason schedule for the 2013 season were released Sunday night. More ...
Buffalo tried an onside kick, but Nate Ebner recovered for New England. Blount then ran 10 yards before scoring his second touchdown on the next play.
The game even featured a punt by Brady on third-and-32 from his 46-yard line midway through the third quarter with the score 16-3. It traveled 32 yards, where Jim Leonhard made a fair catch.
"It could have been a lot better," Brady said.
NOTES: Bills tackle Marcell Dareus was benched for the first half for violating a team rule. In Buffalo's previous game he was benched for the first quarter, also for violating a team rule. ... The previous Patriots record for all-purpose yards was 273 by Larry Garron in 1962 at Buffalo. ... Carpenter's 51-yard field goal that tied the game at 3 made him the first player in Bills history with four field goals of at least 50 yards or more in a season.
Copyright 2013 by The Associated Press