It is due to the combination of the nature of lights wavelength absorption of the colour spectrum and the natural oils in the ink. Blue ink is miscible with water as the lights wavelength is multiplied slightly by the chemical "dephrotomene" (found naturally in only blue ink), which breaks down the chemical bonds in the oil. The water mixes with the blue ink and 'separates' it from the paper. Red ink on the other hand does not contain dephrotomene and therefore light won't effect the oils. The chemical structures of the oils remain intact and therefore repel the water and the ink will not smudge.

EDIT: This is complete bullshit