When you store building blocks from a document, the only templates, other than the attached template, the Building Blocks.dotx file, and the normal template, in which you can store building blocks are templates which already contain at least one building block and which are loaded as an add-in or in the building blocks folder. If you want to use a custom template for storage, either as a global template or a building blocks template, it must have at least one building block in it already (created when it was the attached template or open for editing directly.