Looking southeast over the Coastal hills and Richardson Bay to Sausalito, Alcatraz, The San FranciscoÐOakland Bay Bridge and San Francisco at evening twilight. Taken from from Mount Tamalpais State Park (Marin County, California, USA) in the Coastal Range, on a rare, crystal clear January night after a winter storm cleared the air in the Bay Area.
California State Route 1 is seen in the immediate foreground.
Visibility was over 60 miles.

The north tower of the Golden Gate Bridge is seen on the right.
The blue color on the land is created by a fading blue sky after the sun has set.

According to Wikipedia:
"Twilight is the illumination of the Earth's lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that the surface of the Earth is neither completely lit nor completely dark. The word "twilight" is also used to denote the periods of time when this illumination occurs.

"The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, twilight's brightness is nearly zero, thus evening twilight ends, and night begins. When the Sun again reaches 18 degrees below the horizon, night ends and morning twilight begins. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the bright sky, twilight has long been popular with photographers, who refer to it as 'sweet light', and painters, who refer to it as the blue hour, after the French expression l'heure bleue.