After a quiet opening 15 minutes, the visitors scored the opener after Adam Armstrong found Lewis Holtby, who neatly slotted past Cameron Dawson.
Wednesday’s poor start got worse when Massimo Luongo saw red in the 23rd minute following a high challenge on Lewis Travis, who then deepened the Owls’ first-half nightmare when his initial shot was brilliantly tipped on to the post before rebounding off the frame of the goal, back off Dawson and into the net.
Their collapse was punished further with Holtby’s second of the game just before the break when he got on the end of another Armstrong cross to tap home.
It was more of the same in the second half, with Darragh Lenihan and Sam Gallagher adding two more to complete a dominant Blackburn showing.
Monk said: “I didn’t see it coming at all, it was a horrible day for us. It’s difficult to analyse straight away, but the main problem is that it seems like it’s always two steps forward, one step back.
“I think today was self-inflicted, our biggest mistake was the first 10 minutes of the game. We speak about wanting to play on the front foot early in the game and playing in their half, but we turned down that opportunity.
“We kept turning back out, putting team-mates into trouble and inviting needless pressure and mistakes.
“That set the tone for the game, obviously the red card doesn’t help but I’m not going to use that as an excuse. The problem came before that and we got punished today.
“We’ve come off the back of some good performances and good results, but we seem to shoot ourselves in the foot. There’s work to do but that’s my job to do that.
“But we will stick together, the players and I take the responsibility but there’s football to be played and we’re in a good position despite what happened today.”
Blackburn manager Tony Mowbray expressed how pleased he has been with his young side’s recent showings and felt the victory was coming.
Mowbray said: “Our recent performances have been pretty good, we’ve worked hard on the training ground and today the fruits of that came in the game.
“We understand they played with 10 men for over an hour of the game but the first 20 to 25 minutes we were pretty dominant and on the front foot. But obviously the extra man helps take the sting out of the crowd.
“We managed to keep them quiet and got the job done. We don’t let anyone play out from the back, we’ve scored the most goals in the first 15 minutes of the match and that’s the way we play. It suits this group of players.
“It’s a pretty young team, we have a lot of 19, 20 and 21-year-olds playing in that team. I think we showed that we can be a threat in this league.
“The attacking side of our game was good today, our defensive side of the game and the concentration was exceptional and we didn’t really give them anything.
“We didn’t come here thinking we were going to win 5-0 but we always come believing we can win. We know their team and we know their players.”