The Ordo Sesteria, or "Order of the Seven" is a secret organisation founded during a troublesome time for Blackrock, to maintain peace between the exterior districts of the growing city.
Its original founders came together, after riot had threatened the inhabitants of all districts, in squabbles over where the borders between districts should be drawn and how they should he allowed to govern themselves.
A lot of these conflicts at the time were precipitated by the official ruling class of the districts and the governance of the city as a whole. The seven founding members who stepped up, where themselves anchors of their communities, who sought to bypass official channels and to take cooperation between the districts into their own hands.
By addressing issues within their communities, trading favours to enable cooperation, bribery and other, more unsavoury means, they managed to calm the situation down and to bring the city back from the brink of civil war, within its borders. An act for which the city official council did, of course, take credit.
The locals in the districts however, knew who had really helped them. As a result, thus uneasy alliance between those with power within the districts remained, still seeking to maintain Blackrock's delicate balance. Over time, this cooperation led to many of the city's great advancements, with a lot of the infrastructure projects managed in the subs, that mainly help another district, being guided by the Order. Likewise with magical breakthroughs heralded by the universities in the skies above the city, in the Dreamlands.
Unlike back when it was first founded, the identities of the seven are kept secret nowadays, and have been for a long time. Both for their protection, and because the reach of the Order has expanded with the city, with many people being brought in as agents, or freelancers, hired to work in the interest of the Order, and to champion its causes.