The novella is written in epistolatory form, specifically as a series of military dispatches between an emperor and his governor on the frontier. The title refers to the two colours of ink used in those messages: purple for the official dispatches, and black for the personal communiqués enclosed with them (the book itself will be printed in those two colours; I can't tell you how it looks, because my proof copy is in monochrome -- but, if past experience with Subterranean books is a guide, I'm sure it will be gorgeous). It also serves as a metaphor for how outward appearances can mask the truth, just as the formal tone of the dispatches conceals the reality to be found in the longer messages.

The story goes that Nicephorus was a scholar, until he suddenly found himself made emperor when all the male members of his family managed to do each other in. Needing to surround himself with people he could trust, Nico sent his best friend Phormio north to be governor of Upper Tremissis, where he now has to deal with unknown insurgents who appear to have inside information.

The epistolatory mode is an interesting choice, as it takes the focus away from the "action" as such, and turns it more on Nico's and Phormio's travails. The banter between the two friends rings true, and makes them more convincing as characters in a story that doesn't necessarily give its author much more opportunity to develop them.

Actually, I think Parker deals well in general with the limitations of her chosen structure. She manages to tell a complete story that gives due acknowledgement of the impact its events have on the characters who live through them (even if the distance of the telling means we don't necessarily experience that impact so much ourselves). She weaves in the back-story skilfully, and raises some difficult ethical issues. Here, for example, is Phormio ruminating on what it is to command his forces to go after the enemy:

In one sense, you're the Angel of Death. People are going to die because of you, and isn't that the most appalling thing imaginable? On the other hand, it's perfectly all right, because the bad guys are the enemy, it's like killing rats, not homicide but pesticide; and some of the good guys will have to get killed, too, because that's the price we pay. Well, not you and me personally. Just soldiers. It's what they're paid for.