Hey
Yeah this can be a bit of a problem, as is the same with relying on any one metric to judge something by.
As great as SEOmoz PA/DA etc. are they are probably more easier to game than say pagerank - this could be by more spammy links being counted, or say if the domain did have a site on, was dropped, then re-registered by someone else with lower quality content or content dragged out of the wayback engine put on it.
Overall I would always look at a blog and think 'if I was interested in this content would this be the kind of site i'd read ?' - if the answer is yes then go for the guest post, if not then maybe move on.
Other metrics such as social (facebook fans, twitter followers, G+1s etc.) can also help give an idea of how genuine the site is, although there are also ways to game these as well.
Hope that helps!
Stuart