A fine solo effort from Coutinho just before half-time at Old Trafford led to Jurgen Klopp's men equalising on their way to securing a 1-1 draw and 3-1 aggregate victory in the last 16.
Blind said that was the decisive moment of the second leg, which United led thanks to an Anthony Martial penalty.
"It was a big blow for us, of course," he told Uefa . "We were aggressive after the first goal, we were on point, we were sharp with the crowd behind us.
READ MORE | Keane: Man Utd play like a bunch of strangers
"I had the feeling there was more to get, and then it's a big blow that you concede a goal just before half-time."
Coutinho beat United full-back Guillermo Varela before delicately chipping David de Gea from an angle moments before the break.
The hosts still had their chances, but Blind said his team's rivals did likewise as the game opened up in the second half.
"You have to be honest, in the end Liverpool had chances as well," the Dutchman added.
"We take risks in the second half, the spaces get bigger and Liverpool get chances as well.
"We were attacking and the spaces were bigger in our defence, but you know if you need to take a risk and score three goals, it's a risk you>