Pretty sure this would be considered a bug. While running in the Editor, change the default UI Image Material color, it changes it permanently in the project. I’m of the opinion that the default Material, seen in the Editor Image Inspector window as, “None (Material)” should be unalterable, or reset to Color(1,1,1,1); after the Editor run completes.
C#
v 2020.1.4*
Mac
Cups of coffee: 4
// This will make all your UI images black if they’re using the default Material, “None (Material)”
GetComponent().material.color = new Color(0, 0, 0, 1);
// This will make all your UI images invisible if they’re using the default Material, “None (Material)”
GetComponent().material.color = new Color(1, 1, 1, 0);
// And just because I can, this will make my pink unicorn, invisible:
GetComponent().material.color = new Color(1, 0, 1, 0);
IF for some reason you have stumbled upon this post, and you just want to fix your stuff. Stick this in a Start method in one of your UI scripts:
GetComponent().material.color = new Color(1, 1, 1, 1);
Run it once, and you should be good.
Then comment it out, because you don’t need to use it anymore. Hail Hydra.