"She is absolutely gorgeous. We were all in love," Williams, 31, told USA Today at Saturday's BET Honors in Washington, D.C.
Beyonce, 30, Jay-Z, 42 and their little one have not yet been photographed since the much-reported birth went down in a posh, private "executive suite" at Lenox Hill Hospital two weekends ago.
But the superstar couple "have already settled into their role of being parents, and it's just awesome to see," Williams added.
Also at the BET event, ex-Destiny's Child member Kelly Rowland was more reluctant to dish on the most talked-about baby of the new year.
"That will be something that everybody will have to experience the same way I did," the "Commander" singer said. "That's for her parents to disclose, not myself."
Little Blue's birth was "emotional and peaceful," Beyonce and Jay-Z said in a statement last week. "We are in heaven."
MORE FROM WONDERWALL:
- Sep. 24, 2016 Pippa Middleton wants Adele to sing at her wedding!