This is an easy one. Downspouts are usually next to concrete foundations and having the water flowing over a concrete shield...alkaline. The lime in concrete affects the soil near the foundation raising the pH.
This is the worst place for acid plants. I understand, I think, that you thought lots of hydrogen atoms that would make the soil more acidic. What kind of roof material do you have?
I would make a plant bed elsewhere and add sulfur to that soil to lower the pH a little more. Do you have a pH meter or two? There are also fertilizers made for plants that do better with the pH down to 6 or 5.5. Mulch with peat moss, very acidic (but kind of a no no as peat bogs are being depleted). Lowering pH is tougher than raising it I am afraid. Those plants will be fine in neutral or 7.0 and you can continue to add sulfur until you are at the pH you want and then just maintain that level just for that plant bed.
Chemistry is everything. Chemicals make up all the world and life around us. Sulfur is sulfur. I'd be aware of fillers, however. The only chemicals I know that are bad are pesticides, fluoride, chlorine, lead, arsenic those kind of obvious 'bad' chemicals. Not fertilizers or lime or sulfur.