What is the exact meaning of “blood-dimmed (tragedy),” and how does it pass current among Anglophones?

Solution 1:

You'd probably raise some eyebrows if you used blood-dimmed in normal conversation. It's not at all a common usage, and I'd hazard a guess that even most native speakers wouldn't be too clear on the exact meaning (it's dimmed = made dim, dark, akin to rivers dark with blood).

I also doubt most people would recognise the allusion to Yeats’ poem, but it certainly seems to me most references to blood-dimmed in Google Books are followed by the word tide, and do in fact stem directly from “The Second Coming”.

In this case I think it's probably fair to say Dowd is aiming more for a sense of "scholarly erudition" than "flowery language". She probably knows most of her readers won't pick up on the reference, but she assumes the few that do will admire her for using it (and themselves for "getting it").

Solution 2:

SUPPLEMENTARY:
I think FumbleFingers not only hits the nail on the head, he drives it pretty much home. This is just to countersink it by addressing this piece of your question:

I wonder why Dowd [...] doesn’t plainly say ‘the tragedy of 9/11 was blood-chilling / curdling’ instead of ‘the blood-dimmed tragedy of 9/11 was chilling.”

Whatever her use of it, Dowd has a pretty firm grip on how the language works; so I think she says blood-dimmed rather than blood-chilling because that's what she means. Dimmed means made not just dim and dark but also obscure, difficult to make out; Dowd is suggesting that W's fear was aroused not just by the horror of the event but by its opacity.