They're showing support for the ACLU, which says the ribbon is a way for stars to stand up "for the rights of everyone as guaranteed under the Constitution."
"Loving" star Negga kicked off the trend among the stars, wearing the pin on her red, flowing, long-sleeve gown.
"They're a big part of our film," Negga said of the movie that chronicles the love story of Mildred and Richard Loving, an interracial couple who fought for their right to be married.
"I'm wearing an ACLU ribbon because they're fighting incredible fights right now for American ideals."
The little, blue ribbon was the perfect pop of colour for model Kloss's white, floor-length gown.
Stars have shown increased support for the organization in recent months. During her acceptance speech at the Screen Actors Guild Awards in January, Sarah Paulson had urged the public to donate to the non-profit group.