This has been fixed with our hotfix binaries Telerik_UI_for_Silverlight5_2014_2_0707_TRIAL_hotfix (you can download them under Your Account -> Latest Internal Builds). Indeed the issue was the same as the one you pointed out - there should be preset CaretBrush for the TextBox. If you open RadMaskedInputBaseStyle
under ExpressionDark or VisualStudio2013 themes you'll see the fix (these themes require different Caret Brush than the default Black one):
I attached a sample demonstrating that the CaretBrush is White in ExpressionDark theme.
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