The new dark mode actually adopts shades of grey to generate panel contrast, providing a more polished look as a result. You can enable it from the Quick settings menu (the cog wheel icon in the upper right). Currently it only works with the default blue theme.
The dark mode for Outlook.com is one of the most highly requested features for Microsoft's web mail service, according to listings on the Outlook.com feedback site, although there's still no word on whether a similar mode is coming to the mobile Outlook app.
Last month, Microsoft launched a rebranded version of its Microsoft News app for iOS with a dark mode. Several other iOS apps have similar modes, including Twitter, YouTube, Reddit, and Twitch, while Apple's upcoming macOS Mojave also includes dark mode support.