Q: We had a flower pot in the dinning room, on a rug over our hard wood floor. When I picked the rug up I discovered some of the floor has darkened.
A: Sounds like this has happened over a long period of time to cause water damage like this. You will likely have to replace the damaged boards to make this spot go away.
Related Q: I recently over-watered a plant in our living room and it overflowed onto our hardwood floor. Before I noticed it, I happened to go into our garage (which is directly under the living room) and found that the water was pouring out from the underside of the floor. Should water just flow straight through so easily?
A: One would not think so but clearly the water found a way between the boards of your hardwood floor (that means you have pre finished or some significant gaps between boards) and then found another gap in the sub floor; whether your floor is sitting on plywood or is a much older house with pine or spruce plank sub floor.
Similar Q: Can spilling a glass of water on a hardwood floor, yet wiping it up within 5 minutes, cause the floor to crack?
A: I don’t know what you mean by crack, and I assume this floor has some sort of finish on it. I would consider this event insignificant.