The Blues conceded five tries, including two in the opening 13 minutes, as Treviso recorded their second-ever victory over the Welsh side.
Treviso's third win of the season means only four points separate the two sides in the league now and McIntosh admits the reason for the defeat was the Blues' poor defensive organisation.
"Defence is my department and at times our defence was absolutely woeful," he said. "So there's no one to blame but myself and obviously the players because it is my department."
Mat Luamanu and Angelo Esposito both crossed the whitewash as Treviso opened up a 15-point lead within the opening quarter of an hour.
Enrico Bacchin, Henry Seniloli and Jayden Hayward also touched down for the hosts, who managed to secure a try bonus point by half time.
Kiwi full-back Hayward notched up 20 points for Treviso during the game, including three penalties and two conversions.
Lloyd Williams, Josh Navidi and Adam Thomas scored for the Blues as they tried to stage a fightback but they fell one try short of a bonus point.
"It's about the players applying what we do in training," McIntosh continued.
"We do it all in training, we do the tactics, we do the systems and I'm just stumped. I just don't know what happened out there today.
"It is about us regrouping. We are very disappointed. We have nowhere to hide and need to get on with it."
Defeat means the Blues have won only four games this season and lie in ninth place in the GUINNESS PRO12 table and McIntosh admits European qualification is now going to be a tough ask.
"A lot is going to depend on other results," he said.
"We have to remain positive, roll our sleeves up and focus on winning a large percentage of our remaining games."
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