Poison while related to darkness, is more akin to hive magic, and we don’t have any hive magic abilities. (Outside of lore) The elements we use are the purest forms of each individual power. While there may be light and dark power respectively, the elements seem to be applications of this power upon the real world, thus explaining how we are able to produce them when we are wielding both Light and Dark respectively, as proven by Prismatic. Hive Magic and Arcana is another application of Darkness, but it is diluted in comparison. It’s magic which uses darkness as a fuel, as opposed to the direct application of darkness itself made into a tool, much like the elements. In other words, instead of Darkness being made into the object, Darkness if fueling the process to then make the tool, that being Hive Magic. So poison is darkness fueled, but not darkness itself.

At this point, poison will remain an exotic gimmick.