Try this
<UserControl.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/PROJECTNAMESPACE (TestProject.Something);component/Dictionaries/Styles.xaml" />
</ResourceDictionary.MergedDictionaries>
<BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter"/>
</ResourceDictionary>
</UserControl.Resources>