Potassium nitrate is nothing more than "stump remover" sold in 1 pound containers at the local home center ( Lowes, HomeDepot).
It's all I've ever used, gives a nice deep blue/black.

Niter Blue

Scroll down to the 5th post.
More Niter blue

HTH, Robert

Last edited by HIGH$TRAP; 07/16/11 08:32 PM.

The clearest way into the universe is through a forest wilderness.
-John Muir