Don't give a damn about YPC, because it's a stat that is only useful when the situation is taken into account. The Patriots look to get most of their yardage in the air. Half the time when they used Blount they only needed a yard or 2. If you use a guy predominately in short yardage situations, he's going to have low YPC despite doing exactly what you wanted.
The Patriots don't usually run for field position, so YPC isn't a great stat to track a Patriots style running game. Belichick did a lot of running with Blount on first or second and goal, and second or third and short yardage. Break the plane and move the chain. Or hell, just keep the clock moving on a time-chewing drive late in a close game. Plays that are necessary to success, but tend not to give a lot of yards. You could have given a lot of his snaps to any decent running back and they'd be OK at them, but Blunt was actually particularly good at prosecuting those snaps.
Hell, 18 touchdowns should tell you how frequently Blount was the man on the goal line and how much of his role was to convert on short yardage plays. Blount was very good at that for us, especially at exploiting teams with bad D against the run or exploiting teams that planned their D too much against Brady. But it won't give you a ton of yardage despite being an absolutely necessary and valuable contributions.