"Black leaf" seems to refer to either a derivative of a walnut tree which is smoked like mj, or to nicotine, which is somewhat toxic. The website below is from Cornell's med school, which I assume is reputable, and gives information as to the toxicity of nicotine. http://pmep.cce.cornell.edu/profiles/insect-mite/mevinphos-propargite/nicotine/insect-prof-nicotine.html

Fortunately, this horror has lost it's MOA license a long time ago, as it had a nasty tendency to kill more greenhouse farmers than mealybugs. Nicotine had the huge combined advantages of a almost magical efficacy and a very short residual toxicity in the environment. A naturally occuring alcaloid, it was also permitted in some instances of Biological Farming, as an environmentally friendly, and "natural", pest control agent. What you'd do with it would be, like, apply it immediately before harvest on i.e. greenhouse tomatoes, knowing that it would have broken down completely 2 hours after application. Farmers would don adequate protection, and spray, or fog the formulation in the greehouse, then exit, close the door, go off to lunch, re-enter the greenhouse to proceed with harvest, and... never come out alive. The two hours were, in fact, a theoretical limit after which the Nicotine would have decayed sufficiently in order not to hurt any new whiteflies flying into the said glasshouse, and in practice it was much, much safer to spray early in the morning, lock the door, and wait for the Sun to destroy the Nicotine until the next day. Haste can kill! Nicotine at "high" doses (50 mg as opposed to a ciggie's 2 mg) is very poisonous, kills very quickly, and has no specific antidote. As an assassination weapon, it is also fairly deniable if well aimed and dosed appropriately, as no environmental residues would probably be found around the victim, and the Cotinine and such like Nicotine metabolites could be expected to be found in the corpse's body fluids HPLC analysis, in quantities not much different from these ordinarily seen in a smoker's, assuming that the forensic people would have had the good idea to go look for them in the first place.