Boys gotta get their rest!
The Cruel Intentions star, 39, and her designer boyfriend welcomed Arthur on July 25 in NYC. The newborn weighed in at 7 lbs, 12 oz.
"I feel like the luckiest girl in the world," the actress told Us Weekly at her June baby shower, hosted by BFFs Sarah Michelle Gellar, Jaime King, Karen Zambos and Janey Lopaty.
Blair -- who ended her two-year marriage to Ahmet Zappa in 2006 and dated her Kath and Kim costar Mikey Day from 2008 until 2010 -- announced her pregnancy in January.