I could be way off here, it looks like you’re trying to override virtual functions from the original UAnimGraphNode_BlendListBase, but you haven’t included the override specifier at the end of the declare in your header file.
ANIMGRAPH_API virtual void AddPinToBlendByFilter() override;
ANIMGRAPH_API virtual void RemovePinFromBlendByFilter(UEdGraphPin* Pin) override;
That might clear things up for you, as the linker could find issue with these functions as you are altering their definitions in the child class, which inherits their original definitions from their parent class. Let me know if this helps resolve your issue.