Oh, this is fabulous! First off, the opening is great. You already reveal so much about them and their unsentimental, never-meant-for-happily-ever-after affair in those few sentences. Your writing is excellent, and the characterisation is fantastic. Love what you do both with Rita and with what little we get about Mrs Zabini in canon – sharp tongues, ambition, intelligence, a good sense of their own advantage, and especially in Thea's case, her suffering from double standards, both as a black woman (who isn't treated like other purebloods) and as queer woman (she preserves her reputation, he gets to indulge). I also like how you chart Rita's progression along the milestones of her affair with Thea, the story of her hair colour, her career movements, her other affairs, and how we, like Rita, never really learn much about the husbands.
Crouch’s newest deputy strikes her as particularly promising; Amelia Bones has the look of a woman whose flinty exterior might yield under the right approach. -- Ah, doesn't she just?
Her confusion over whether “Blaise” is meant to be a boy or a girl’s name helps her justify the impersonality of the gesture. *g* at this, and the awkward sentence constructions!
“Building your reputation on the back of a dead man and his secret affair again, I see. You never cease to astound, Rita. – W. Grubbly-Plank”
Finally, I insist on full details in any number of sequels you choose to write on Gwenog and the shower rooms and Wilhelmina the excellent letter-writer who eventually goes Minerva-centric.