You arrange the order by positioning the series in the series collection, which is why the order that you add them is significant, however the default orderings of these are working at cross purposes for your goal.

The best way to get the effect you want, currently, is to manually manage the legend items. This way you can set the order however you want. Here is an example of how to approach this:

        <igChart:XamWebChart x:Name="chart">
            <igChart:XamWebChart.Legend>
                <igChart:Legend Width="100" >
                    <igChart:Legend.Items>
                        <igChart:LegendItem Fill="Red" Text="Bar2" />
                        <igChart:LegendItem Fill="Blue" Text="Bar1" />
                    </igChart:Legend.Items>
                </igChart:Legend>
            </igChart:XamWebChart.Legend>
            <igChart:XamWebChart.Series>
                <igChart:Series x:Name="bar1" 
                                ChartType="Bar" 
                                Label="Bar1" 
                                Fill="Blue">
                    <igChart:Series.DataPoints>
                        <igChart:DataPoint Label="A" Value="1" />
                        <igChart:DataPoint Label="B" Value="2" />
                        <igChart:DataPoint Label="C" Value="3" />
                    </igChart:Series.DataPoints>
                </igChart:Series>

                <igChart:Series x:Name="bar2" 
                                ChartType="Bar" 
                                Label="Bar2" 
                                Fill="Red">
                    <igChart:Series.DataPoints>
                        <igChart:DataPoint Label="A" Value="3" />
                        <igChart:DataPoint Label="B" Value="2" />
                        <igChart:DataPoint Label="C" Value="1" />
                    </igChart:Series.DataPoints>
                </igChart:Series>
            </igChart:XamWebChart.Series>
        </igChart:XamWebChart>

Hope this helps!

-Graham