I answered your support threads but still I will paste the answer here as well:
In order to remove the black border appearing on the selected RadOutlookItems, you will need to edit the control template of the RadOutollBarItems in Blend and remove the OuterBorder_Selected BorderBrush from the SelectVisual Border.
"-1 0 -1 -1"
I attached an example illustrating this approach.
Also you can find more information on How to edit control templates in Expression Blend here
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