Blue Hour times
Blue hour is the short period of time before sunrise or after sunset when the sun is just below the horizon. Indirect sunlight is evenly diffused and takes on a blue shade. The duration of the blue hour varies depending on the location, but generally lasts less than an hour.
Generally, the best Blue Hour shots have sources of artificial light, such as street lamps or city lights. This warm light contrasts the dark blue, giving your photos a wider range of color. Without this light, your images could end up looking too blue or colorless.
The final glow of the sunset or the first rays of sunrise can also add color to your photos. To get the most out of this faint light, try to shoot somewhere with water. Then, the sunlight will reflect on the water, giving you twice as much color. This strategy works especially well with large bodies of water, such as the ocean or a big lake.
If this has excited some interest then this article https://www.photopills.com/articles/blue-hour-photography-guide will take you further into "blue hour" photography.