PFF does not throw out any plays. They grade each play on the traditional coaches grading system of -2 to +2 in increments of 1/2. If a guy has 9 plays graded 0 and one +2, they don't throw out the 0s and call him a +2 player.
It's worth keeping in mind that in applying those grades, PFF makes purported informed assumptions about what a player's assignment is on each play. A play grade is "normalized" for game situation, however that's suppose to work, and a player's season grade is adjusted for how consistent his game grades are over the course of a season. Consequently, as they note, it is possible for a player to get a season grade that is higher than any of his individual game grade.
As for that unblocked blitzer, there are a range of negative-to-positive outcomes for that player. On the negative side, the player could trip and fall, whiff badly on a dead-to-rights sack, or drive the QB into the ground for a 15 yard penalty. On the plus side, it might range from chasing the QB out of the pocket to actually getting that clean sack. To my mind a 0 play would be chasing the QB who's in a called bootleg the other way to a short throw without actually hurrying him.
One wonders how they grade a CB play were he blows a coverage and the QB doesn't throw the ball that way. The examples given focus on actual outcomes. I would presume that would get a negative grade even if we never see it on a telecast, though I couldn't be certain.
It should be noted that nowhere anywhere in the PFF stuff I've read, and I've read a lot of it, does it say adjustments are made for level of competition. An edge rusher going against Bakhtiari presumably does not get a break. Whether that competition colors the invidual play grades that are assigned by "experts" who know who's on the other side of the ball is a question.
As you can see in the link below, a well-executed short throw by a QB typically gets a +0.5. I would expect a well executed sack on an unblocked blitz woud earn at least the same.
https://www.pff.com/pff-player-grades