Make legend item of BarSeries reflect it's visual style. For example, if I'll make a BarSeries with a border like this:
<Style TargetType="Border">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="BorderThickness" Value="1" />
    <Setter Property="BorderBrush" Value="{StaticResource chartRed}" />
</Style>

then there won't be a rectangle in a legend as expected (it will show a transparent one instead, without border).

REASON FOR DECLINATION:
The RadLegend is a standalone control which allows for customization of the legend item visuals (one can use the ItemTemplate property of the RadLegend to control the legend item's appearance or can retemplate the LegendItemControl). There is no direct link between the ChartView and the RadLegend control and the appearance of the legend elements can be done by framework means such as Style, DataTemplate and ControlTemplate. The suggested way of manipulating the look of the chart legend item is making use of the ItemTemplate property of the RadLegend and the MarkerGeometry property of the series legend settings.