AFAIK, Blueprints cannot be used directly in C++ as types unless they are already derived from a C++ class. I’m fairly certain this includes passing a reference as a C++ parameter.
EDIT: If the blueprint in question is a UMG widget, this is not the case. You need to add some references to your Build.cs, and you will have access to Slate and UMG. If it’s not, then the above should hold true.