Comment by simonh

Case in point being the Black Chamber run by the Thurn und Taxis post in Brussels in the 16th Century.

https://link.springer.com/chapter/10.1057/9780230298125_11

Signals intelligence intercept and analysis centres have been called Black Chambers for a long time, including the first such group in the US, predecessor to the NSA:

https://en.wikipedia.org/wiki/Black_Chamber