Building Blocks are stored in templates. By default, they are stored in one of the building blocks templates, but you can choose to store them elsewhere. As stated, you want to use a custom template for your document (not normal.dotm or one of the building blocks templates that come with Word). Distribute that template to your users. They do not open it, but rather use it to create a new document. It can be placed in their user templates folder or a workgroup templates folder.

Templates in Microsoft Word
Boilerplate in Microsoft Word

You may want to use Greg Maxey's Building Blocks utility.