It rather depends on the topic, if there is no redefinition of the topic used, the DP system uses the default value of the corresponding DependencyProperty , which is Brushes.Black .
There are, however, many possible overrides in the legacy of the Aero theme changing meaning in some cases. While the TextBlock style does not set Foreground directly (which would be a more powerful override), Foreground of Windows set to {DynamicResource {x:Static SystemColors.WindowTextBrushKey}} , so if the TextBlock is in the Window , the brush associated with this key will be applied if inheritance is not canceled by another value of even higher priority .