Chocolate blends often contain soy lecithin. Lecithin is an emulsifier (think: mustard binding an oil-and-water salad dressing, or egg yolks in Hollandaise). Soy in chocolate helps bind the cocoa solids, sugar, and milk so they stick to cocoa butter a little easier. This improves the flowy-ness of the chocolate when it is melted down and put into temper.
We recently switched to chocolate blends that use sunflower lecithin to do a similar job! However, some of the finishing and decorating components we use have trace amounts of soy. We are currently working on finding solutions to remove even more soy from our menu, so stay tuned! For more info on specific ingredients we use, check out this article: Are Bons allergen-friendly?