It is meant to increase acidity and lower hardness as well as "soothing" properties which a lot of people believe in.
The blackwater extract is just expensive extract, I would use Alder cones or IndianAlmondLeaves which would give the tannin tinted look as well as good water chemistry altering which you would want with the species mentioned... far cheaper and more effective option.
Where would I buy Alder Cones or Indian Almond Leaves? Just online?