ahha maybe u should ask mother nature that.. it's just like
that! no need to do anything one.
i call it the "twilight hour"
the sky looks very very very dark blue to the naked eye during that
time everynight (in sg it's about 730-745 zhun zhun) and when u
take a shot it comes out slightly brighter, becos of the long
exposure.
just take a shot as u would any other night cityscape scene...
typically with a long exposure of 4-30 seconds depending on ur
aperture.
for us digicam users, it'd be around 4-8 seconds at F8 max (cos our
cameras can't reach a smaller aperture)
note: small aperture (small hole) equals BIG F number... 
generally speaking:
small hole (big F number), more focus area (greater of depth of
field) for landscape shots.
big hole (small F number), less focus area (shallow depth of field)
for portraits (background will be blur)
