I can't tell you what a good black salve product might be, but I can tell you from experience to be very cautious and to avoid black salve products with high zinc content. Those high zinc black salve products appear to be quite caustic and react to just about anything, including clear skin.
I found out the hard way, when I decided to use some black salve to see if any of the suspicious spots on my face might be cancerous. Instead of following instructions and trying it on just two or three spots at once, I decided to put it on anything which was not clear skin (wrongly thinking that if it did happen to be cancerous it would react and otherwise wouldn't) - which turned out to be about 15 different spots. For two days, it felt like I had fifteen constantly lit cigarettes being pressed into my face, and there were even some places where I accidently brushed the stuff on clear skin that reacted.
It wasn't pretty or pleasant (to understate greatly):
Now, years later I still have scars present - though greatly diminished.
As an alternative to black salve, one might try a skin creme based on highly concentrated oleander extract. It works much slower, but with persistence will clear up all kinds of skin "thingies", including warts, moles, pre-cancerous lesions, age spots, blemishes, fine line wrinkles and more.
Do-it-yourselfers can find complete instructions for making a safe oleander extract at home and how to further condense it down and make a skin cream here:
Note that the instructions for making the most effective version of the skin cream call for including iodine. Some have suggested that since iodine is so reactive as well as since repeated skin painting with iodine can tend to blister a bit that it might be best to apply it seperately (alternating between applying the skin cream without iodine and applying iodine).