The actress agreed to wear an eco-friendly black gown, designed by Carolina Herrera, to the Metropolitan Museum of Art's glitzy Met Ball, while The Office star donned a Tom Ford tuxedo.
Blunt's dress was made from recycled wool from the 1940s and '60s, while Krasinski's tux was also fashioned from ethically-sound materials.
The actress says, "I'm so excited that Carolina Herrera has created this gown for me with elegance and ethics in mind. It's gorgeous and edgy and it thrills me to be a part of a fashion moment that hasn't compromised itself in creativity yet has the important story of using all sustainable fabrics and techniques."
The star couple wore the outfits as part of Colin Firth's eco warrior wife Livia's Green Carpet Challenge (Gcc).