Well, look at how a similarly conventional team like Memphis was taken apart piece by piece despite first team all defense players on Curry and Klay both(Mike Conley, Larry Allen). Cavaliers stayed very smart on defense in doubling Steph off screens all series and denying Klay the ball and they stayed disciplined, forcing Iggy/Barnes/Green to brick a lot of important shots.
On offense, it's not like he had options. I thought the only miscue was a reactionary decision to not play Mozgov at all in game 5. The rest was on point, despite the clown narrative.
So no, based on this series, I wouldn't agree that Blatt is terrible.