Not really an apples to apples comparison - when Peterson gets to the postseason, look at who his QBs have been. During his 2000 yard season, he dragged a team that had no business making the playoffs into the playoffs, and for the wildcard round his quarterback was... Joe Webb. Joe Webb cannot throw a football to save his life. Joe Webb completed went 11/30. Joe Webb is now a kick returner. So naturally the Packers stacked the box all day since the only way they could possibly lose that game was if Peterson ran for 200+ yards.
And I think that's the main point. Throughout Blount's Patriots career, nobody spent the week leading up to a game determined to stop Blount, under the understanding that Blount was the engine driving the Patriots' offense. Blount saw very few stacked boxes in his time here. To his credit, I can think of a few games when Brady was suspended last season, and maybe an Indy game or two, where that might have been the case, and when it was he delivered. But the overall point--that having a good QB makes a RB's life much, much easier--still remains. Put prime Adrian Peterson on any of these Pats teams for their playoff runs, and the teams are significantly better. Put prime Adrian Peterson on the 2015 team's playoff roster, we'd be looking back at a threepeat. Put Blount on some of those Vikings teams that Peterson dragged to the playoffs, and they're probably going 5-11.
Having said all of that, I do agree with your first point. Blount slowed down and was a lot less effective as the year went on, but he was a huge part of going 3-1 in the first month. No way we could've done that without him, and he was a big part of the team last year. At this point I'm not sure I'd take Peterson over him, either. Peterson's still an unknown; he might be 90% of what he used to be, or he might be 50% of what he used to be. I'll take the known quantity in Blount if I have to choose between them. And I'm sad to see Blount go because I like the guy, and I'll miss seeing some of the fantastic, punishing runs he delivers, but I think it was the right move.