"I'm pregnant!!!" Clarkson, 31, tweeted Tuesday. "Brandon and I are so excited! Best early Christmas present ever."
She did not mention the baby's gender or anticipated due date in her Twitter message.
UsMagazine.com said Blackstock has a daughter Savannah, 11, and son Seth, 7, from a previous relationship.
Clarkson and Blackstock were married last month at Blackberry Farms in Tennessee.