Since no one has responded with an intelligent answer, I'll hazard a
guess...

Perhaps your water is basic (high pH) rather than acidic (low pH). I'm
wondering whether this is the same reaction you see in cooked red/
purple cabbage, which can turn blue if the pH is too high (vinegar is
often added to keep the red color).

Just a SWAG,

Alan

piya wrote:
have been drinking a ?cooked pu-erh... chun ming tuo with blue lettering on
the wrapper....

after brewing, and drinking, the leftover tea turns a blue colour after some
hours...

when drunk, the stuff looks and tastes fine...

why ???? (should I continue to drink it ??)

thanks, Andrew