You could try ammonia or white vinegar, but I did find the following....
"Removing blue stains on bathroom fixtures.
Blue, green or other unusual colored stains, can be caused in old, badly worn bathtubs and sinks, from cleansers not rinsed off thoroughly. When harsh abrasives (such as most scouring powders) are used regularly on these porcelain enamel surfaces, over several years they wear down the smooth surface, leaving it rough. Dyes used to encapsulate the cleaning particles in scouring powders or other colored cleansers can get into the crevices of such worn surfaces, and if not rinsed off, and allowed to stay and harden for days, can be almost impossible to remove. A strong chlorine bleach solution may lighten the stain, but will also further dull or discolor the porcelain surface. So one may have to live with the stain.
This article was written by Anne Field, Extension Specialist, Emeritus, with references from the Porcelain Enamel Institute."
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local