The icon is the same music logo, but with black space around it instead of white space. It’s a live trademark application and awaiting examination. It’s currently the logo for Apple Music for Business, but I wonder if we could start to see more dark mode icons.
Apple’s trademark filing showing dark Apple Music.
The mark consists of a black square with rounded edges with a blue, purple, and pink stylized musical note in the center.
This tradmark registration is intended to cover the categories of production of programmed music and video displays for subscribers; audio and video recording and production services for others; music service, namely, providing specially programmed background non-downloadable music for retail establishments, public areas, and commercial establishments via telecommunications networks, computer networks, the Internet, satellite, radio, and wireless networks; musical, radio, television and video entertainment services, namely, custom arrangement and editing of music, audio and video programs; Custom music programming services.
Last year I wrote an article where I made mockups of iOS dark mode icons, and I hope Apple considers it for iOS 14.