Getting rid of black shadow/stain in toilet caused by interaction of iron sponge with anticalc + toilet gel with bleach
Try Iron OUT or similar cleaners with sodium hydrosulfite and/or sodium metabisulfite, which reduce iron and manganese stains, rather than bleach (Javel) which does the opposite. Place a paper towel over the stain and soak in the remover, so that it does not run off immediately. Let it sit for some hours and repeat as needed.

Do not use abrasives such as pumice, or hydrofluoric acid (HF), as in Whink Rust Stain Remover, which will remove the stain, but will also permanently damage the china bowl... which your landlord may ask you to replace.