There could be several reasons why your well water is turning black.
The concentration of iron or manganese is getting higher in your well water. These minerals can cause the water to appear black or brown in color. Organic matter or sediment is building up in your well, which can discolor the water. If you are living in an area with a lot of organic matter, such as decaying leaves or soil, this organic matter might have dissolved in the well water and caused it to turn black.