In researching the Culinary Chronicles for World Wide Recipes, I turn up some pretty peculiar things. For instance, and of particular interest to this mystery/food group, is the adulteration of sweets ingredients with dangerous additives during the 19th century--a widespread problem. The bright colors that appealed especially to children were achieved by the addition of mercury, arsenic and lead, among other poisonous materials. Sand and POWDERED GLASS were sometimes added to sugar and brick dust, red lead, chalk and sulphate of lime was added to cocoa powder.
Candy that isn't poisoned can still be dangerous to candy factory workers, including causing tooth decay and silicosis just from inhaling the sugar in the air, allergic reactions and citric acid burns from orange and lemon juices.
As for adding poison to candy deliberately, there's a long tradition of that in the mystery canon. A mean thing to do. A wicked thing to do. Almost as bad as sending somebody an all-cream assortment with absolutely no nut clusters.