I was a little too distracted to participate in the blogathon, but I would contribute two brief notes. First, the trajectory of scholarship on noir is similar to how Geoff Mayer characterizes the scholarship on "pre-Code" Hollywood: use of concept, followed by revisionism, followed by a return to the concept. In the case of noir, the return has fallen into two camps, the "yes but" approach of a James Naremore, who acknowledges the incoherence of definition but still thinks there is something to a popular crime modernism in 1940s Hollywood, and the approach of not engaging with the incoherence-critique. As far as the critique itself, Marc Vernet is the most polemical in denying film noir as an idée fixe, and Thomas Schatz's (and others') notion that noir really comprises distinct genres is an insight that does not get enough play. It's possible I'm overlooking some other good interventions in the field.
Second, 1947 is a banner year for noir (James Naremore cites it as the peak in noir production), so while I've not watched all of the 47 films or even 47 noirs, I've now had a good cross-section of them. What is striking is that the characterization of 47/the immediate postwar as a noir-heavy time does hold up, since noirs outnumber pretty much any other genre, except for maybe B Westerns. (I know this sounds odd to write, but I do think in other respects the historical picture of the period has been quite distorted by the canon and by film availability.) At the same time, not all that is low-key is film noir, and I keep coming across titles (Daisy Kenyon or Nora Prentiss, say) that get labeled noir by tenuous criteria. Moreover, Schatz's point holds: many of the noirs have an ur-genre (gothic, police procedural, detective story) that better fit them.