The biggest problem I have with Bo's method is there's no consistency. The criteria for earning them doesn't seem set in stone, and it appears he uses them as a motivational tool. Unfortunately, it often doesn't motivate anything.
Look at how the team has earned them, and what happened after they earned them, through Bo's tenure.
2008
Earned: Blackshirts were handed out after the 10th game, a victory over Kansas where we gave up 35 points and 422 yards. A fair portion of those points/yards came in garbage time, as Nebraska led 31-21 in the fourth quarter before both teams' defenses coasted and each scored twice more.
Next Game: A victory over Kansas State where we gave up 28 points and 247 yards. The Huskers won their last three games after receiving the Blackshirts.
2009
Earned: Blackshirts were handed out after the 6th game, a loss at home to Texas Tech where we gave up 31 points but only 259 yards. This was the game where the Tech QB (I forget his name) threw up half a dozen prayers and his WRs made circus catches. Tremendously lucky game for Tech.
Next Game: A loss at home to Iowa State where we gave up 9 points and 239 yards. This was the infamous 8-turnover game where everyone fumbled and Lee threw costly picks. The Huskers won six of their last seven games after receiving the Blackshirts, the lone loss coming against Texas in the Big XII CCG, the infamous 00:01 game.
2010
Earned: Blackshirts were handed out after the 4th game, a victory over South Dakota State where we gave up 3 points and 236 yards. This was a very ugly game, you'll recall, where the Jackrabbits had several concerning drives. Error-filled on the part of the offense, though.
Next Game: A victory over Kansas State where we gave up 13 points and 315 yards on Bill Snyder's birthday. A parting gift as we left the Big XII for Eric the Red's favorite team.

The Huskers went 5-4 to finish the season and lost three of their last four. The defense gave up an average of 18 points per loss, but the offense was terrible.
2011
Earned: Blackshirts were handed out after the 8th game, a victory over Michigan State where we gave up 3 points and 187 yards. Nobody can say the defense didn't earn their Blackshirts after this game as they stifled a decent Sparty offense, holding the Big 10's best WR, BJ Cunningham, without a catch.
Next Game: A loss to Northwestern where we gave up 28 points and 468 yards to a team that finished the season 6-7. We couldn't stop a simple crossing pattern all game long and appeared disinterested on defense. Nebraska went 2-2 to end the season, including losses to Michigan and South Carolina where we gave up 45 and 30 points.
2012
Earned: Blackshirts were handed out after the 8th game, a victory over Michigan where we gave up 9 points and 188 yards. The cynic will say that Denard Robinson only played half the game and his replacement, Ron Bellomy, was inept, but Robinson had only rushed for 46 yards (10 carries) and passed for 55 yards (6 of 11) prior to his injury.
Next Game: A win at Michigan State where we gave up 34 points and 361 yards in a very hostile environment. The defense played well enough that game, but they allowed Sparty to move the ball with ease. Nebraska finished the season 3-2, giving up 70 and 45 points in back-to-back season-ending losses to Wisconsin and Georgia.
Over Bo's tenure, without the Blackshirts we're 27-9 (.750 winning percentage) and we give up an average of 20.4 points per game.
With the Blackshirts we're 21-11 (.656) and we give up an average of 22.2 points per game.
The one caveat to these stats is that the easier non-conference schedule is included in the "without Blackshirts" schedule, skewing the numbers slightly in favor of "without."
This is the problem as I see it - there doesn't appear to be any motivation the team derives from earning their Blackshirts. They don't appear to improve, they don't have any great swagger, and they don't appear to carry the pride of earning those Blackshirts onto the field. Of our six worst losses in the past two years (2011: Wisconsin, Michigan; 2012: UCLA, Ohio State, Wisconsin, Georgia), three have come with the Blackshirts and three without.
There is no discernible difference between the defense's play with or without the Blackshirts. There is no discernible merit to "making them earn it," judging either by wins/losses or points/yards allowed.
It's a distraction. It becomes a story line where no story needs to exist. It becomes one more thing an already-struggling unit has to think about.
That's my reasoning for getting rid of Bo's method. Just give them the Blackshirts after Fall Camp and let them get down to the business of playing the game.