When I was up in Pennsylvania last year, I went to King's Herb Nook just a little north of Gettysburg and got a little jar of Black Drawing Ointment and it is some powerful stuff. A friend of mine was bitten at the base of her thumb by a brown recluse (she saw the spider so it's not 'probably' a brown recluse) and her doctor kind of shook his head and told her the relative horror she was in for. I gave her some of the BDO and she bandaged it onto her hand and THE NEXT DAY the bite was almost healed. I told her to go show her doctor but don't know whether she has or not. It's really excellent stuff. I think I have a recipe for it at home; if I can find it I'll post it.
Here's the place I bought that little jar of it. It's a family-run thing, and I can tell you they must be doing something right. There were two girls who waited on us, and they had absolutely the most beautiful skin I've ever seen on a human being. Very nice folks -- chickens right behind the shop!
Did the Black Drawing Salve that you bought from King's Herb Nook list the ingredients? "Black Salve" is a wide category. Some of the ones I've seen are charcoal & clay based similar to the "black drawing ointment" recipe you posted. These are helpful in pulling venoms and other toxins out of the affected areas.
Many black salves are made with Bloodroot (Sanguinaria). I believe Dr. Christopher's black salve origininally called for Bloodroot but that ingredient was changed to Poke due to the controvery over Bloodroot.
We use a bloodroot type of black salve for removing moles, warts and skin tags. This type of black salve has been in use for centuries and is considered an escharotic drawing salve (meaning it creates a scab as as the first stage of healing) and has to be used correctly to avoid problems.