With the sun below the horizon, the only illumination is sunlight scattered through the upper atmosphere — and that scattering strongly favours blue. The sky glows evenly rather than casting directional light, producing a soft, shadowless, distinctly cool wash.
It is the best window in the day for city work. Ambient sky brightness falls close enough to artificial lighting that street lamps, windows, and signage register at the same time as the sky, instead of one blowing out while the other goes black. Architecture and skylines look their best in exactly this balance.
It is short — typically twenty to forty minutes — and it demands support. Light levels call for slow shutters, so a tripod or a solid surface is effectively mandatory. Auto white balance will often try to neutralise the blue that is the entire point, which is a good reason to set white balance manually or shoot raw.