Hrm... Question for the TL, because I genuinely can't remember: Is the RAW of "Cinderella" the Katakana, or is it something like "Ashkaburi-hime"? Because, if the latter, please preserve that, for two reasons...
1) It's fun for people to learn that "Ash-Covered Princess" was the original name for Cinderella, and 2) It's also quite literally her name for the good reason of smearing ash into her hair to hide the colour.
Anyway, yeeee, the assassin arc starts in earnest!
This is a screentone effect, used to make a gradient. Specifically, this is called kakeami, and it comes in several flavours, such as noughts and crosses, clawmark hatching, and, indeed, The Boxes. There's a lot of different screentone types, not just the dots (which were originally for saving ink at the printers.)
Basically, it's a form of shading largely unique to manga.
EDIT: And props to the people who can actually work with it. I've tried, and it's an absolute sonuvabitch, with no tutorials I can find on how to do it well...