Hi Richard,
Yep --- a mystery. The stain was noticed on the steps and skimmer boxes when we lowered the water level about 3' (1500 gallons) working on getting the CYA down. (it's working slowly but-----). Like I've mentioned the liquid bleach was the only (apparent) change.

I pour the bleach in at the return stream about a foot from the edge at the three jets. Then stand on the diving board and spray (pitch) the remainder out across the surface of the deep end of the pool.

The stain is not localized so I doubt it is a result of direct contact.

I'm making preparations to do the stain removal (L-Ascorbic Acid) and metal chelating removal process (Versine Acid EDTA [from the MSDS]) . I just can't figure what chemicals are part of the reaction that caused the stain(which is why I posted here).