Beyonce Looked "Unbelievable" at Bash With Jay-Z

Beyonce and Jay-Z attends the 52nd Annual GRAMMY Awards held at Staples Center on January 31, 2010. Credit: Kevin Mazur/

Beyonce and Jay-Z are back in an Empire State of mind!

Following a super-luxe getaway on the celeb-friendly island of St. Bart's -- their first vacation with daughter Blue Ivy, 3 months -- the singer, 30, and rapper, 42, made a surprise appearance at a private Cartier event at Skylight Soho in downtown Manhattan on Thursday night.

Leaving Blue Ivy at home, the superstar couple surprised singer Rita Ora, a protegee of Jay-Z, who was performing the Destiny's Child classic "Say My Name" when she realized her VIP pals were present. "I"m embarrassed!" a happily shocked Ora admitted to the crowd when she spotted Blue's mom and dad, a source told Us Weekly.

Clad in a beautiful white top, "Beyonce was doing a little dance" to avoid being seen, the observer tells Us, adding that the singer's hubby "smoked a cigar" in their private section. "They were very sweet together . . . They seemed to really have fun!"

Another reveler says that the fresh-faced singer appeared more stunning -- and slimmer -- than ever. "She had on no make up, or very little," the observer said. "She looks unbelievable. She seems to be about a size 6 now.”

The "Love on Top" singer was in a more reflective mood earlier in the week, when she posted a handwritten letter to First Lady Michelle Obama on her website

"Michelle is the ultimate example of a truly strong African American woman," the singer posted on "She is a caring mother, she's a loving wife, while at the same time, she is the First Lady!!!!"

Added the star in her touching letter: "Michelle, thank you so much for every single thing that u do for us -- I am proud to have my daughter grow up in a world where she has people like you, to look up to."