"I'M ENGAGED!!!!!" she tweeted Saturday. "I wanted y'all to know!! Happiest night of my life last night! I am so lucky and am with the greatest man ever."
People.com said the 30-year-old singer began dating Blackstock, the son of Clarkson's manager, Narvel Blackstock, a year ago.
The elder Blackstock is married to country music icon Reba McEntire.
2014: The Year in Music [PHOTOS]