Look at the absorption spectra at http://omlc.ogi.edu/spectra/hemoglobin/index.html. Green is ~510nm, red is ~650nm, blue is ~475nm. The difference between the absorption of Hb and HbO2 in the red range is the reason why pulse oximeters use red. In this case, we don't care about differentiating oxy- vs. deoxy-hemoglobin, we just care about the total absorption. Since more light is absorbed at green than at red, it will show more of an effect with pulse.

I am not confident in this explanation, and still don't know why green over blue.