My 20-month old son had these same sxs 3 months ago (excessive hunger and thirst, constant diarrehea, and blistery rash).

I know how frustrating this can be. You feel your child's pain. They finally found, through a blood test, that he was intolerant to both gluten and milk. The dark stool usually indicates blood in the stool. My son had such a painful, blistery rash. His little tush would blister upon contact with this stuff. I thought that I had my son on a gluten free diet, but there are so many things in food nowadays (see the forbidden list on this website). At least 200 things that are gluten-containing. At first, I was giving him Rice Krispy treats and Rice Chex, not knowing that they contain gluten. The forbidden list is worth printing out. Do NOT trust restaurants!

I am finding that out the hard way. French fries and other foods that you would never think would have gluten, do. I make everything up from scratch these days, this way I know that it is gluten free and rice milk (not cow's milk) going in the recipe. Anyways, best of luck to you.