For the moment, I don’t think there’s a way to access the materials from the content folder in Blueprints.
It’s probably possible in C++, but I don’t know how exactly. From what I’ve seen, Materials are C++ classes, since they can’t be modified without recompiling.
You can get all objects of one class, but not a list of classes.
Provided that it’s possible to do this in C++, you could write a method that returns a list of material classes and manipulate that in blueprints afterwards.
BTW, what are you using the materials in the array for? Maybe there’s another solution.