While cooking dinner I had an idea inspired by the Nitrazine yellow for titration thread.
Black beans as pH indicator, you cant get less toxic than black beans
I saved some of the bean soaking liquid and chilled it. Cooling precipitated some starch or protein, so I filtered that out. I then diluted by half and buffered aliquots of the liquid to various pH values.
In acid conditions black bean tea turns pink, at neutral its grey or purpleish, at basic pH it turns green.
Another, more traditional, food-safe pH indicator is purple cabbage juice which gives a whole rainbow of pH values. Black beans just tell you what you need to know: green = add acid, grey or purple = just right, pink = too much acid or not stirred well enough yet.

As a bonus, my black beans tasted good

Lovely. I could have saved some bucks instead of bying the nitrazine