The relativistic doppler effect can be subdivided in 2 parts. The newtonian part and the pure relativistic part due to time dilation or length contraction.

http://en.wikipedia.org/wiki/Relativistic_Doppler_effectLongitudinal velocity component:

The effect on the frequency in the velocity direction for the newtonian part is greater than 1 for an approaching speed (1+v/c) and smaller than 1 for a receding speed (1-v/c). For the purely relativistic doppler frequency shift, it is equal to the gamma function and it is thus always greater than 1, or blue shifted.

Transverse velocity component:

For a transverse velocity, there is no newtonian shift, but the relativistic part is now 1/gamma, always a redshift. This cannot be a discountinuous function so any transverse component will introduce an additional redshift!!!

If the excessive redshift was due to dust, it would be visible by scattering effect (blurring effect).