What I would try, in this order:

1. Pressure washer

2. Chlorine bleach.

3. Hydrogen peroxide (or Oxi-Clean)

4. Rit Color Remover (sold where the Rit dyes are) (Sodium hydrosulfite)

5. Oxalic acid solution

Good luck & please let us know what, if anything works.

I've had pretty decent results removing stains from concrete with TSP (trisodium phosphate). It can be hard to find locally, but you can still get it online. Try not to get "TSP Substitute."

Make up a strong solution with hot water (maybe 1/4 cup TSP per gallon), scrub with a stiff brush, let stand, scrub again, rinse. Repeat if necessary. Wear rubber gloves; TSP is rough stuff.

30 Second Outdoor Cleaner.