mac mail blacks out messages with dual monitor setup on mackbook air
This is a common issue on High Sierra Mail. It’s caused by a compatibility issue with some of the graphics cards. I was able to fix it by changing the Color Profile.

Go to System Preferences>Display>Color and then switch to a different profile until you find one that both looks good and fixes the problem.

You can also try and use the calibrate feature to create a custom profile.

For more info on how to change the Color Profile: https://support.apple.com/kb/PH25231?locale=en_US

More info: 
https://discussions.apple.com/thread/8135091