
Originally Posted by
Unknown008
if that stain is due to copper sulphate, you can try adding aqueous zinc solution to it.
Also, if it's copper sulfate (or sulphate, if you prefer), ammonia will form a very soluble amine complex with copper. You can easily wash it out (it will turn deep blue after it forms, but that won't bother anything). However, I doubt that bluing agents, in general, contain copper. I think they're typically organic chemicals that absorb the yellow. However, since bleach won't get it out, it's definitely possible that it's an inorganic salt.