What is a Tokyo Raven? I don't recall the title of Tokyo Ravens being explained over the course of the anime (though it's possible I missed something). Other than the Raven Coat, I don't remember any particular references to ravens. Where does the term Tokyo Raven come from and what does it mean? Is there a historical basis for it or is it something the author made up? Is it explained in the light novels?
Where does the term Tokyo Raven come from and what does it mean? 


The correct parse is almost surely Tokyo [Ravens] rather than [Tokyo Raven]s. As in "ravens in Tokyo"; not "more than one of a Tokyo Raven". 

"But what ravens?" you might ask. "I sure didn't see any in the anime!"


  Is it explained in the light novels?


Yep. It turns out that the light novel adorns a number of phrases with the furigana レイヴンズ "ravens". For example, from the back cover of the first volume (to use the most obvious example, since I don't have this OCR'd and hence can't Ctrl+F through it), there are already two distinct instances of this: 


[陰陽師たち]【レイヴンズ】 ("shamans" or something, depending on your translation)
[闇に舞う鴉たち]【レイヴンズ】 ("crows who dart through the darkness"). 


There's not a one-to-one equivalent of this practice in English, but a reasonable approach is to understand it as "ravens" being used as metonymy for "shamans"/etc. And this is true in-universe, too: shamans are sometimes referred to or described as "ravens" because of their tendency to don black, raven-like cloaks, as Yakou did. 

This furigana thing occurs multiple times in the actual body text of the LN (in volume 1, at least), but it'd take me a while to hunt down the actual quotes. There are also direct (non-furigana) uses of "ravens" (鴉) as metonymy for shamans. 

Of course, there are many shamans (i.e. ravens) in Tokyo; and many "crows who dart through the darkness" (i.e. ravens), if by "crows" we mean "weird shadowy supernatural things". Hence, Tokyo Ravens - ravens in Tokyo.



But your confusion is reasonable: this would probably be lost on people who have only seen the anime, since the furigana double-meaning thing obviously doesn't work with the spoken word. 


  Is there a historical basis for it or is it something the author made up?


I know very little about real-world onmyoudou, but I'm not aware of the raven being a motif or symbol there. (Of course, research is made difficult by the fact that Tokyo Ravens itself pollutes google results for this sort of thing.)