I have attached a small project based on the code you provided. As you can see in the attached image, the marker brush is gray, as is the fill of the area series.
In your case, I do not see the fill of the area series, it seems to be transparent. This is why the marker brush is also transparent - by default the fill of the legend marker is bound to the fill of the area series. I am unsure why the fill of the area series is transparent in your case, perhaps you have some settings in code behind. If you were to remove these settings, the area series should display with the color from the palette and the legend marker will be correctly applied. If you need the fill of the area series to be transparent, you can use a line series instead. Another approach would be to set an item template for the legend where you pick the color of the marker by some custom logic.
Let us know if I was able to address the issue at hand properly and whether you need more information.