I don't think that such single-Publication/Repository "BluePrints" without Structure Groups are ever used in practice.
Once upon a time there was this idea to sell/license different Editions of Tridion R5 and one of the differences between those Editions was the maximum allowed BluePrint depth. I think the cheapest Edition (targeting Small Businesses) had a maximum BluePrint depth of 1, meaning you can only have top-level (root) Publications and not actually have BluePrinting at all. I would say a rather silly license model for a system which has BluePrinting as its Unique Selling Point, and I don't think such Editions were ever actually sold.
Note, however, that even this extremely resticted license did allow you to create Structure Groups in those Publications (otherwise you also can't really publish much, making them even more useless :-)
Note that I'm sometimes referring to Publications as Repositories. You also find both concepts in the CM domain model (Core Service and TOM.NET). The idea is that all Publications are Repositories, but not all Repositories are Publications.
That is: the term Publication suggests that it is used for publishing, but that is not the case for all Publications in a BluePrint; you can argue that a BluePrint consists of several Repositories and some of those are Publications (typically the ones lower down in the BluePrint).
The distinction between Repository and Publication does not surface in the UI, though. Nor is it currently possible in the CM API to create a Repository which is not a Publication (class
Repository is currently an abstract base class).
Nevertheless, if you ask me how I would call a Publication without Structure Groups (which is probably not used for publishing, unless you only publish DCPs), I would call it a Repository.