The Savages beauty welcomed the tot at a hospital in Manhattan, New York, a source tells the New York Post's Page Six gossip column.
No further details about the child's birth were available as of yet, and a representative for the new parents has yet to comment on the news.
The newborn is a sibling for Blake and Ryan's daughter James, who will turn two in December.
Meanwhile, the family's new arrival already has friends in high places - the insider reveals Taylor Swift was en route to visit mother and baby on Friday morning (September 30).
The pop superstar was among the guests at Blake's baby shower, which doubled as her 29th birthday bash, in the Big Apple in late August, when the actress gathered friends and family at the Campagna restaurant at the Bedford Post Inn, which is owned by Richard Gere.
It was not known whether the couple was expecting a baby boy or girl, as the party location was decorated with gender-neutral white and green balloons.
The news of Blake's pregnancy first surfaced in April, and she debuted her growing baby bump at the world premiere of her Woody Allen film Cafe Society at the Cannes Film Festival in France in May.
Blake and Ryan have made no secret of their desire to have a big family.
"I'm one of five kids; my husband's one of four," Blake explained during an interview on U.S. breakfast show Today in June, before joking, "We're officially breeders! You can go on our website and we will give you some of our children."
The couple wed in 2012.