“Miner’s powder” has been in widespread use for a century – but, as the name suggests, for earthmoving. It has been employed occasionally by daring sappers in intractable sieges, but otherwise has not been put to military use.
Because of its volatility, igniting black powder with Theurgy requires barely any blood, and causes an explosion whose scale is extremely hard to control or predict. Deploying it in pretty much any form against Theurges would be suicide, and as Theurgy has been the determining factor of nearly all military conflicts for 300 years, there has been very little incentive to develop powder-based weaponry. Theurges themselves prefer to use oil of vitriol for their battlefield pyrotechnics, as it’s easier for them to stay in control of the resulting fireball.
If firearms were invented, Alastors could in theory put them to use, as they’re highly unlikely to face Theurges. However, for a variety of reasons the Thaumatarchy prefers to keep its internal security enforcers armed primarily with iron-shod clubs.
PS: On this issue as any other, I welcome anyone who wants to poke holes in the internal logic of my world…