"I hope you know how thrilled I am that you are one of us now," the late-night host told the British-born star on his show Tuesday.
"Really?" Blunt, 32, asked. "Did you not like that I wasn't before?"
"There was a wedge between us, I felt like, yes," Kimmel said, adding, "We always get the sense that when people come here from England they kind of look down their noses at us."
The actress joked, "Oh, we do! Yeah! Very much. Because we're better than them. But now I'm only half better."
Blunt then confessed that she found the whole process of becoming an American citizen "strange and slightly disarming."
"I'm not sure I'm entirely thrilled about it," she said. "People ask me about the whole day. They were like, 'Oh, it must have been so emotional.' I was like, 'It wasn't! It was sad!' I like being British."
She then went on to describe her swearing in ceremony.
"It was the most bizarre day. Matthew McConaughey's wife [Camila Alves] was getting sworn in with me, who is so nice," Blunt told Kimmel. "McConaughey shows up looking like he's going on safari. I was like, 'You could have worn a shirt and tie. Like, Camila looks amazing,'"
The actress added that her husband, actor John Krasinski, later confessed to her that he was doing his best not to stand next to McConaughey and say, "All rise, all rise, all rise" -- a turn on the Oscar winner's famous phrase.
The most horrible part for Blunt, though, was having to renounce Queen Elizabeth II.
"It wasn't specifically Queen E, but she knows," the "Sicario" star actress joked.
She went on to explain, "The thing that's weird is I do get to keep both my British citizenship and this, but you have to renounce her. So, it's kind of typically American -- not to be rude -- but I had to renounce her in the room but I don't actually, technically renounce her. They were like, 'Just say it. You don't have to mean it, but just say it.'"
Kimmel then asked Blunt about the U.S. citizenship test she had to pass before getting citizenship.
"It is the hardest test I ever had to take," Blunt said. "I guarantee most Americans would have no idea how to answer any of these questions."
Blunt obviously passed, but how did she do when Kimmel gave her his version of the citizenship test? Watch to find out.