"...The blue hour comes from a French expression, l'heure bleue, which refers to the period of twilight 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. The blue hour is considered especially flattering for people with blond hair in photography and is often also when the smell of the flowers is at its strongest during the summertime....."
(from Wikipaedia)