<Border Height="50" Width="200" Background="Blue">
    <Border.OpacityMask>
        <LinearGradientBrush StartPoint="0,0" EndPoint="1,0">
            <GradientStop Offset="0" Color="Blue"></GradientStop>
            <GradientStop Offset="1" Color="Transparent"></GradientStop>
        </LinearGradientBrush>
    </Border.OpacityMask>
</Border>