Thank you for the response Dono!

I did a little more research and that was kinda what I was thinking. There is a low spot of swampy woods by our pond and a lot of that water drained into the pond. We were able to eventually build a dike and keep the water from running in, but there is where the color came from. Maybe we will just add the black dye so it looks a little more appealing without turning a weird color.