Building Blocks are stored in templates. The templates can be stored in the building blocks folder; they can be stored elsewhere. The default template for Word 2013 is named "Built-In Building Blocks.dotx."

Building Blocks Storage Location

I don't store any custom building blocks in the building blocks folder. Doing so is asking for trouble, as is storing macros in the normal template. I store them in Global Templates, that way I can also assign keyboard shortcuts or QAT icons to them.