[Frankenstein]

In case someone is looking for the same answer: the Omni Group's Joel Page told me that there is no way of doing this as the Blank template resides within the application itself. Instead, he suggested I create a new document, make all the modifications I want, save the document (it needs to be in ~/Library/Application Support/OmniGraffle/Templates, as I have found out subsequently) and then tell OmniGraffle, through the preferences, that I would like to have this new file as my default template.

Once again, thanks for your help, Joel!