b-r.com lists JJJ as having a 3.6% block percentage. For comparison, Stiemsma is at 8.5%, Jermaine O'Neal was at 5.8%, and KG is at 2.5%. Kendrick Perkins has a career block percentage of 4.7%. Garnett is at 2.9%. Jermaine O'Neal 5.0%. Shaquille O'Neal 4.6%. With JO waived to make room for Sean Williams, JaJuan Johnson is now statistically the second-best shot-blocker on the team.
Small sample size, obviously, but Johnson's numbers suggest he could be perceived as an above-average but not great shot blocker, enough to keep the opposition honest but not enough to intimidate. Players in the 3.4-3.8% block percentage range include Darko Milicic, Josh Smith, Derrick Favors, Ed Davis, Tyson Chandler, and Joakim Noah. If you give the valid criticism that Johnson has a lot of garbage time minutes, players in the 2.8-3.0% block percentage range include Kendrick Perkins, Channing Frye, DeMarcus Cousins, and Travis Outlaw.
The last time JJJ blocked a shot was March 23 when he had two in the game, blocking Thaddeus Young, who got an offensive rebound after his five-footer was blocked only to get blocked again by Johnson.