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