I know what you mean. It is not a paint. Ive used it before multiple times. It also stains skin bad (well, just about anything).
If bleach doesnt work, maybe conc. H2O2 (but this is very reactive, and good luck getting a hold of enough).
Ummm, oh , ya, maybe try some muriatic acid from the hardware store. The main purpose of this stuff is to clean bricks and concrete. I'd imagine muriatic acid (conc. HCl) will get below the stain, and eat a thin amount of concrete below it, and then the stain will be gone.
Now that i think of it, im very confident muriatic will work. Its like 4 bucks a gallon at the hardware store (although, you can get quarts of it).
Just realy follow instructions on the bottle, becuase muriatic is some strong stuff.
If that doesnt work, take a power sand, and put the toughest grit sand paper you can find (like 60 grit will work, otherwise the grit is like stones glued to cardboard) and sand the hell out if. Might work, but i think muriatic will work good.