Funnily enough, only days after I’d posed this question on my blog, noir author Kate Laity guest posted elsewhere on exactly the same subject. You can see her erudite replies over at Mollie Cox Bryan’s blog. She puts it so much better than I ever could!
Tess Makovesky 
Thanks for sharing, Tess. That really is a thorough and thoughtful discussion of noir. As both of you make clear, in a way it’s a bit nebulous. But you both outline it very well.