My family has a cabin in Black River Falls, Wisconsin. The Black River as well as most of the lakes, streams, groundwater are blackwater.
Their color comes from tannins leached into the water as water flows through marshes, swamps, and areas with root systems of plants containing tannins.
It's safe to drink and swim in, and doesn't stain, but it also isn't purely black. Much like tea, put it in a
clear cup and it'll appear to be tinted brown. Put it in a cup a black coffee mug and it'll appear as black as night.
On a
clear day, you can usually see a few feet deep into the river and stream-beds. More than that and you'll only see black. Less than that and it may deceptively appear crystal
clear.
When I say it doesn't stain, what I mean is that in the concentration it is in in blackwater, tannic acid won't stain a swimsuit. Used in higher concentrations, it's used to stain wood -- it gives wood finishes their dark, rich color.
The summary of all of this is that blackwater sources do exist, but short of getting your water directly from a blackwater lake or stream, you may not want to try using it unless you are dead certain you can achieve the correct concentration. Even then, you may not get the desired results in a shallow depth -- your first
line of defense will always be having a black liner in your pool and using side-lights instead of down-lights.