Mail App bug? Dark Background + Dark Text on random messages (illegible), inconsistent
Do you have have macOS 10.13.x High Sierra on your Mac? There is a bug in the video chip or driver for some models in that version of macOS (also possibly in Mojave) that renders messages black in Mail in some scenarios. I have only seen it once, and then it affected all messages in threads, meaning it only manifested with the Mail View menu -> Organise by Conversation turned on, and then only for messages where there were actually two or more messages in the thread.

The solution, if this is the bug that bites you, is to open the System preferences, go to the Displays pane -> Colour tab, and try another one of the Display profiles in the list (uncheck "Show profiles for this display only"). Some will display the bug, some won't, so try to find one that don't show the bug and is pleasing to the eye. Alternatively you can calibrate the display to make a custom profile, and see if that one is bug free.