It seems after Matplotlib 1.3.1 (1.4.1, 1.4.2, not sure about 1.4.0) that setting the transparency of the facecolor or edgecolor properties of a legendPatch is not respected. Both properties can be changed together using legendPatch.set_alpha(...), but they cannot be set independently. Following is a MWE. Here, the legend's facecolor is changed, but the alpha specification (0.2) is ignored. A call to legendPatch.get_facecolor() confirms this.

import matplotlib.pyplot as plt

plt.plot(range(10))
leg = plt.legend(['Line'])
leg.legendPatch.set_facecolor([0.5,1,0.5,0.2])