indigo, any blue (plant) dye
靛 centers on the deep blue color of indigo dye, specifically referring to the plant-based pigment or any similar blue dye.
靛 combines 青 (blue/green) with 定, which likely contributes the sound. The character is used specifically for indigo dye.
The left side 青 gives the blue color, and the right side 定 suggests a settled, fixed dye. Picture a vat of deep blue indigo dye, the color fixed into cloth.
For テン, imagine ten (テン) indigo-dyed cloths hanging to dry: ten -> テン, and the deep blue color is unmistakable.