How to Clean Black Pool Grout?

[gigito2]

Upon opening our pool this year, we see that the grout surrounding and around the tiles is BLACK! We tried scrubbing with chlorine using a brush attached to a drill. No improvement.
Does anyone have any suggestions or product recommendations? TIA
Just an FYI, we have an over-abundance of maple leaves and the cover was "slimy" this year - a first for us, as is the grout problem.

[Teapot]

Strong chlorine bleach will do it if you give it some time, pool strength not the weak one from Walmart.

[gigito2]

Thank you, Teapot. I bought some from the pool store today & will try tomorrow. (Raining now.)

[gigito2]

I just wanted to thank you again, Teapot. Used the super chlorine from the pool store as you suggested and it did a fantastic job! Tiles and grout are almost sparkling.

[Teapot]

Thank you for taking the trouble to pop back and let us know.