Would a druid realize if these are unnatural or natural? Is it a magical charm making otherwise healthy blue mushrooms look this way, or is there an actual corruption of the plant, or is it an entirely new species?

Good question! This is not a new species. They are the same mushrooms as the old blue mushrooms in the good ol' days. However, the kobolds have coated these mushroom in shadow using a magical charm. It bars the blue glow, could restrict underdark creatures from feeding on the blue mushrooms, and maybe simply look corrupt. On the other hand, it doesn't seem to be killing the mushrooms! (There are other factors for and against, which you're free to continue asking on.) Ultimately, whether or not coating a mushroom in shadow is corruption is up to your druid to interpret.

Thank you greatly.