Mr Barr made the comment during a CNN interview with Wolf Blitzer. He appeared on the show a day after visiting Kenosha , where protests have raged in the wake of the shooting that left Mr Blake paralysed and recovering in hospital.
The attorney general attempted to draw distinctions between the police killings of George Floyd and Mr Blake.
"What's different?" Blitzer asked.
That's when Mr Barr suggested Mr Blake had a weapon.
"Floyd was already subdued, incapacitated in handcuffs and was not armed," Mr Barr said. "In the Jacob case, he was in the midst of committing a felony and he was armed. So that's a big difference."
Though Mr Blake did have a knife in his car, there is no evidence available that supports the claim that he was armed or suggests that his intention was to grab his knife and use it against police officers.
During the interview, Blitzer told Mr Barr that, according to Mr Blake's family, the man was unarmed at the time of the shooting.
"I stated what I believe to be the difference," Mr Barr said, not acknowledging his inaccurate statement.
Attorney Benjamin Crump – who is representing both Mr Blake and Mr Floyd's family – and the rest of Mr Blake's legal team issued a statement pushing back on Mr Barr's assessment.
"Attorney General Barr is misinformed. The police officers were the aggressors from start to finish, based on video and witness accounts," they said.
The attorneys said not only did the police use unwarranted force, but they also put the lives of onlookers at risk.
"There was never any point in time when there was justification for deadly force. In fact, there were innocent bystanders in the line of fire when he shot seven times into Jacob's back. At all material times, Jacob's back was to the officers and he never posed an imminent threat," they said. "This was never a life or death situation for the officers."
Mr Blake's shooting is the latest instance of extreme violence used by police officers against unarmed black people.
Despite the numerous incidents in which police kill or harm black people who pose no threat to them, Mr Barr told Mr Blitzer that he does not believe racism is a systemic problem in the US. He did not deny that there was a problem, but did imply that racial bias in policing wasn't widespread enough to warrant action.
"There appears to be a phenomenon in the country where African Americans feel that they're treated, when they're stopped by police frequently, as suspects before they're treated as citizens," Mr Barr said. "I don't think that necessarily reflects some deep-seated racism in police departments or most officers."
He said that racism couldn't be systemic because "there are built-in protections against racists in police departments".
Mr Barr's comments come on the same week that a sheriff in Arkansas resigned after being recorded using the n-word nine times in five minutes after a woman he was with talked to a black grocery store clerk, and the same week that nearly two dozen deputies in Compton and East Los Angeles had to be fired or suspended for alleged involvement in deputy gangs accused of terrorising other deputies and city residents.