MUSHROOMS have NO STARCHES; if they are going dark with the iodine test there is something going wrong with the test. Perhaps there is something in the mushrooms that might cause this reaction, but all mushrooms are SAFE (well, unless they are otherwise strange or poisonous).
Yes, most leafy greens are fully safe, also.
The general rule is cooking vegetables, especially those which grow underground will cause the starch to become more available, whereas cooking (or drying) fruits helps convert the starches into sugars; most dried fruits are safe.
Some things cannot be so easily tested, like caramel which is very bad for us.
And there are some surprising things like tofu made from starchy soya beans is safe; the starch has converted into types of sugars, causing it to gel.
Best approach to using iodine is to take a representative sample of the food and place it on an unreactive surface (paper often has starch added to help color it white), then drop about 1% iodine solution (I use "Atomidine" or "Atomic Iodine," which is edible, but sometimes I use regular tincture, which at 3-6% could be diluted with alcohol for our purposes) onto the sample. Wait just under five minutes and check on the sample. A color change that is slight or in the very light purple range and food is moderately safe (could be "borderline"), however, definitive purple to black and it is poison for us.bon appetit