"This new update has improved notification glanceability with a new layout, which shows more of your messages at a glance", as listed by Google.
Android Wear 2.0 marked the introduction to updating Android Wear-powered smartwatches via the companion app available on the Google Play Store.
As a result, you'll need to make sure your watch is synced with the latest version of the Android Wear app on your phone and let it take care of business - this one won't come directly to the watch like other updates.
As you can see in the pictures below, this black color is found on the quick settings page, the app drawer, notification shade, etc.
Also, as Droid Life points out, the naming for the software doesn't make a lot of sense. LCD and P-OLED models still retain the old grey background it seems. Which means the user can view more of the notification content.
Experts believe that AMOLED screens on some particular brands of smartwatch will definitely benefit from the new settings. Currently, only the Huawei Watch 2 and the Huawei Watch has been confirmed to get the theme. In version 2.8 of the app, the background has been made to be almost black, if not really black, to remedy that. Google has also said that this new dark theme will also be improving the readability on the smartwatches with Android Wear 2.8 installed on them. In the meantime, the software of the smartwatch has gone up to 18.104.22.168612071.