The blue hour comes from a French expression (l'heure bleue), which refers to twilight, the period each morning and evening where there is neither full daylight nor complete darkness. The time is considered special because of the quality of the light at this time of day (also in photography; it is considered especially flattering for people with blond hair) and because in the summer, this is often when the smell of the flowers is at their strongest.

     In Scotland this time of day and the attendant light quality is known as gloaming.

     As a result of the perceived specialness of this time, around the world there are restaurants, theatres and hotels called "L'Heure Bleue," and there is a women's perfume by Guerlain (1912) of the same name. The Norwegian rock band Madrugada (Spanish and Portuguese for early morning) were also named after this time.

 SOURCE : http://en.wikipedia.org/wiki/Blue_hour