Our breadcrumbs are consistently a representation of where you are (ie: we show you how to get back to the homepage from where you are currently located), not where you've been (ie: a click by click representation of the pages you've visited). The "where you've been" idiom has a number of issues, chiefly that it doesn't scale very well if you visit more than a few pages.
One of the big reasons, if not the biggest reason that blogs are different than documents and discussions is that blogs are typically written by a single person and as such, they may very well be and most times probably are relevant across a number of spaces / communities. For instance, my thoughts on engineering and Clearspace at Jive may very well be aggregated by the support space, the professional services space and the engineering space. If my blog 'lived' in the engineering space, it loses some of it's flexibility and in some sense ceases to be mine. It's not a coincidence that people don't live in a space:
I accept with your two points: (1) Positional vs. Strict Trail Issues, (2) Orthogonal = Flexibility
However, the architecture should not impose usability issues. Navigating away without a way to get back is detrimental to the cohesion of a space.
As an alternative to changing the positional crumb trail paradigm, how about a 'where used' sidebar with a clear indication of which space has this blog set as its primary blog? A return link (aka crumb trail behaviour) on this sidebar might not be too onerous for this use case?
This may also have benefits for the author as it would show/confirm where this blog entry is being published. Further benefit may also possible with statistics reporting i.e. not only how many views, but also from which space the interest came from.
I like the idea of a "where used" sidebar. Each of our blogs are associated with a private community. Ideally we would have the option to associate blogs with communities the same way discussions and documents are associated.