The device display is rendered using webrtc. If it is black, then something is probably blocking webrtc connections.
1. Add-ons and extensions
If you are using add-ons and extensions, make sure that none of them block webrtc communications. If so, you need to disable them when using Genymotion SaaS.
2. Google Chrome/Chromium
There are known issues with Google Chrome v89 and below with webrtc communications. If you are using Google Chrome, make sure to use Google Chrome/Chromium version 92 or above.