My only concern with mirror black is that it will stand out like infected dogs' genetalia (sorry Eno) if you're sticking to the earlier mentioned lighter monochrome. However, reflective black was one of the first things that sprang to mind.
Eno, do you know the Coles Myer shareholder office at all? Big black mirror glass block. We used to call it Battlestar Galactica. Silly thing is I can't even remember exactly where in Melbourne it was, cos when I went past it was usually about 10pm at night and I was in a cab on my way home from work. Was looking for some photos of it. Any way, the point is it is (or was) just a big, heavy, black reflective cube sitting in the middle of empty, grassed ground, with nothing to break it's lines.
I think sinister is as much about the shape as the surface treatment Sig. Round is usually warm and friendly, no sharp edges to bark. All the samples you show are hard edge, strong squares. Lots of big sharp corners for people to bump into and damage themselves. A bit like government really.
You could probably mix some early bauhaus architecture in amongst that lot too. I love bauhaus design for its practicality and fitness for function, but the architecture is big and blocky and bulky, unfriendly by today's standard.
While looking for photo of it I found this one...
Another little bit of Melbourne I'd forgotten about. It's a heritage listed shot tower, of the sort that was used for dropping moulten led from to make lead ball shot. It was 'encompassed' by a dome and included in the main court of a shopping centre. Possibilities for the new enveloping the old?
I'm wondering if your government building needs to be the only short, wide block amongst the city. Something that brings a certain visual dischord with it compared to the rest of the city. Something to house the mass of beaurocracy. The corrupt city fathers would, of course, take whatever land they wanted to use for themselves and the needs of the growing city be damned.
Love the way the dome is looking. Combination cathedral dome and coal scuttle helmet.