Jerry;
I am not familar with Art's Belgium blue, but i think it is a re-iteration of the old Herter's Belgium Blue. My first bluing experience was with this blue many years ago. It was a very good blue giving good results & I used it for some time. This is considered as a "Hot" or "Express" rust blue. I never blued on a large scale & after trying the cold rust method much preferred the pace to that of the express type so never went back to it.
The only thing I am aware of as to what's "Wrong" with it is that it does contain a Mercuric compound which is "HIGHLY TOXIC", although very efficent as a rust producer.
