A mysterious teacup ailment

Some months ago, I mentioned finding my current favorite chintz teacup, this $3.33 find from one of the area Goodwill stores. It was stained in the bottom (and now it is again), but I bleached it out and it cleaned right up. But then …
Some days later, I prepared a cup of tea and saw these mysterious black spots  begin to appear …
And these! So now I'm rather spooked and don't use the teacup anymore. I've Googled and learned I shouldn't have used bleach, but the reasoning was only that bleach can damage the teacup, not that it could cause a stain. Now since chemistry was never my best subject in school, I'm wondering if any of you smart people might have an explanation for the black spots on my teacup. Anyone?