parts of other mod UI's, such as voxelmap's minimap, or optifines fps display, are darkened considerably while the key is held down.
tested with and without optifine/other performance enhancing mods.
the server im testing on has not yet updated to the new version of veinminer that allows this feature to be used, i dont know if that plays in a role in it or not. (i dont imagine it does, since this mod is pretty much only client side, but you never know)
edit: forgot to attach images

Oop, I think I might know why too. I'm going to chalk that up to my noviceness when it comes to rendering 😛 I will get this fixed for the next release!