Clarkson, 31, took to Twitter on Monday to announce the gender of her baby.
“We just found out we’re having a girl officially! I knew it! Only a girl could cause this much drama with all this vomiting ha!” she wrote.
Clarkson, who got married last October, announced her pregnancy in November last year.
It will be the first children for the singer and the third for Blackstock, who has Savannah, 11, and Seth, 7, from his previous marriage.