Yep it sucks and a really bad break BUT...it was the absolutely the correct call. It was great to see umps explain the call in a press conference. As a player and umpire myself (slo pitch here in Canada) I have seen this call made, also have had to make it a couple of times.. And yes it is a little bit of a judgement call.....
Intent is NOT required, actually physical contact is also NOT required either... All that is required is for a fielder that is not playing the ball (playing the ball is: fielding it, receiving a throw or making a throw) to obstruct a base runner's path as they attempt advance to the next base. This part is very black & white, where judgement comes in...would the baserunner have made it safely to the next base if the obstruction not occurred.
Middlebrook's attempt to get up is a natural reaction in this case BUT again he obstructed Allen's attempt to get home with this action. He didn't intend to obstruct the runner, but he did. The play at the plate is a very close one but definitely out... Would've Allen made it safety if not obstructed....MAYBE. That maybe is all that is needed for the call to be made. Benefit of the doubt MUST fall to the baserunner when there is obstruction.
So the umps got it 100% correct. I realize that is no comfort for a Red Sox fan. As a fan of the game, all we want is the umps to get the calls correct..they did here.