Not only the Nilgiris, but almost all mountains appear blue from a distance. This is due to a phenomenon known as the Rayleigh Scattering Effect. The light from the sun, due to which we see the mountains, is scattered by atmospheric molecules. The electric field of the incident sunlight oscillates the electrons in these molecules which, in turn, radiate light. Light having shorter wavelength (the blue end of the visible range) gets deviated the maximum from its original direction. Hence, mountains, which are near the horizon, appear bluish when the sun is higher in the sky.
The term Nilgiris is the English equivalent of ‘Nilagiri’, Tamil for Blue Mountains. The strikingly blue colour of the hills is attributed to the clustered bloom of the mass-flowering species ‘Strobilanthes kunthiana’, called ‘Neela kurinji’ in Tamil on the hill-slopes. The funnel-shaped blossoms appear only once in 12 years and are light blue in the young plants and purplish blue in the older ones.