well I couldn't get out of the house with the kids full of pox; so I started digging through my cleaning products... ( OMG, just how many bottles of this 'n that or such & such cleaner, does one need anyway? I think I could open a store...lol)

I don't have anymore magic-eraser, but I figured if that product should work than perhaps another product that cleans up crayon, would also work.
I found an unopened bottle of orange clean ( super concentrate) that I think came with a kit of cleaners from the shopping chanel.
it says that it does , well basically everything... too many things to mention ( but then again, what cleaner doesn't..lol).
I looked for ingredients, but could only see three things mentioned: orange oil, limonene, & surfactants.
you are supposed to dilute the stuff, but I did not. ( all the drips did turn the pool water white as soon as it touched the water, so perhaps it doesn't interact with bleach well.)

I used about 2 Fl Oz all together on a scrubbing pool sponge from walmart.
Took about an hour of scrubbing the 14 foot ( by 2 inches) section of liner. and I destroyed the new sponge in the process, but I am happy to report that those old black stains are gone!

Those stains were at least 5 years old, so I'm really happy that they did come off ( with one heck of a lot of elbow grease).

I wonder if regular orange oil would have worked as well...? ( vitamin C did not)
Anyway, I see that orange oil cleaning products work ( or at least this one type did) & if magic-eraser works as well, then I think the answer may be, " Look for products that remove crayon ".
perhaps the next person with black stains above the water line on their vinyl liner can continue this research by testing out another crayon-removing product & see if that works as well.
Thanks everyone for your imput