When I launch Overwatch™ in Windows virtual machine, it starts with black screen.
Integrated graphics cards are not supported in Overwatch.
In order to play Overwatch a discrete (dedicated) graphics card must be installed into the Mac.
To determine what type of graphics card is installed on your Mac, click on Apple logo on Mac menu bar > About this Mac > Overview tab > System Report > Hardware > navigate to Graphics/Displays tab.
If you have got only card in the list - there is no discrete card installed into your computer.