To be a bit more specific; when you create your shader with Unity's Shader Graph and add a parameter to the Blackboard, it generates a Reference ID (i.e. Texture2D_355C4C5). You can either use that, or rename it to whatever you'd like (it seems to put an _ before whatever you decide to name it). Just use your renamed Reference ID (i.e. _ThisIsRenamed) in your Playmaker node and you should be good to go.
Thanks everyone.