If I understand correctly, you want the FaceAlpha value of each patch in the legend to change according to the fill in the plot.
In order to do that, you need to change the FaceAlpha property of each patch contained in the legend by using legend as follows:
[~,h_legend] = legend('area1','area2');
After you do that, you need to find the patch objects.
PatchInLegend = findobj(h_legend, 'type', 'patch');
This will give you a 2x1 array of patches. The only thing you need to do after that, is set the FaceAlpha for each patch. You can do that by:
set(PatchInLegend(1), 'FaceAlpha', 0.2);
set(PatchInLegend(2), 'FaceAlpha', 0.4);
This code will produce the following figure: