On Wed, 26 Jul 2017 20:21:06 GMT, Wayne Boatwright
wrote:

ISTR, the blue stain is caused by acidic water in copper pipes.

Have you tried a cleansing power like Comet and a brush? Scrub it off
with the abrasive powder.


Those usually scratch the porcelain.

Be gentle once and awhile.

- flush

- sprinkle the stain area while wet

- slight scrub and flush.