Not only Nilgiris, but almost all montains 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 atmosphereic molecules
The electric field of the incident sunlight oscillates the electrons in these molecules which, in turn, radiate light
Light having shorter wavelength gets deviated the maximum from its original direction. Hence, mountains, which are near the horizon, appear bluish when the sun in higher in the sky.