Janet,

Within in 30 minutes of noticing the big, black stain, it has disappeared! This was a huge blackish-purple stain that was very alarming and is now gone.

I will apologize up front for my ignorance regarding pool chemicals(which may be part of my problem, unfortunately!). Here's what I have:

The total alkalinity: 90
Ph: 7.7
Chlorine:2.0
Temp: 78
CYA:not sure what this is...is this the chlorine level?

Also, the water is very clear!

The pool was filled from my garden hose, which I am on a county water supply. I do not know if it has metals. At this point, I'm leaning towards thinking I have an algae problem because the Vitamin C tab doesn't seem to diminish the stain however, a chlorine tab does seem to lighten the stain. Also, my husband discovered (when we were out working with all of this a little bit ago) that the string that holds the thermometer in the pool is green (it's a white string).

Thanks for your response! I am a pool virgin, but am really trying hard to learn this!