What specific effect causes Andromeda Galaxy's light to exhibit a blueshift relative to the Milky Way?

The local gravitational attraction overrides the general expansion, causing Andromeda to approach the Milky Way.

While the universe is generally expanding on large scales, smaller, more intimate scales are governed by the overwhelming force of gravity. The Milky Way and the Andromeda Galaxy (M31) are part of the gravitationally bound Local Group. This mutual gravitational attraction is so potent that it counteracts the cosmic expansion between them. As a result, Andromeda is actively hurtling toward the Milky Way at approximately 110 kilometers per second. When a light source approaches an observer, the received light waves are compressed, shifting their wavelengths toward the higher-frequency, shorter-wavelength blue end of the spectrum, which is termed a blueshift.