"Brandon's totally the one," Clarkson, 30, tells Cosmopolitan for its February issue. "I've never been so happy."
After dating for a year, Blackstock, whose father Narvel Blackstock is Clarkson's manager (and Reba McEntire's husband), popped the question in December on what she called the "happiest night of my life."
But romance was a long time coming for the pair.
"We've known each other for years, but we didn't start talking until the Super Bowl last February," Clarkson says in the issue, which hits newsstands Tuesday.
Though she's thrilled to be engaged, Clarkson wasn't sure when they first went out together. "This is the funniest/worst thing ever: One of my superhero idols is Whitney Houston, and the day she died was our first date," she says. "I was like 'This is a bad omen.' "
That turned out not to be true, but it seems that one thing is: love is worth waiting for: "Six years I was single before this," Clarkson says. "I've never been truly loved like I am right now."