According to Wikipedia (the internet's best source for plausible information):

So, assuming that the full spectrum of light is 670nm wide, and you have a light source emitting a frequency in the dead center of that range (565nm), (that I've done my math right and at 1c a 565nm wave is 530.6THz and 750nm is 399.73THz,) and this doppler shift calculator is accurate, the fan blade's tip would have to have a maximum relative speed of about 98.15e+6 m/s, or a little less than one-third c.