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,