The color blue in a rainbow is commonly referred to as "indigo". It's named after a blue dye called indigo that was widely used in the ancient world.

Example sentences in Engpsh:

The rainbow was made up of red, orange, yellow, green, blue, indigo, and violet.
The indigo color in the rainbow always catches my eye.

Example sentences in Chinese:

彩虹由红色、橙色、黄色、绿色、蓝色（即青色）、靛蓝（即靛色）和紫色组成。
彩虹中的靛色总是吸引我的眼球。