Using a LinearGradientBrush
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
HorizontalAlignment="Center" VerticalAlignment="Center">
<Rectangle Width="80" Height="60">
<Rectangle.Fill>
<LinearGradientBrush StartPoint="0,0" EndPoint="1,1">
<GradientStop Color="Black" Offset="0" />
<GradientStop Color="White" Offset="1" />
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
</Page>
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