Explaining the semicolon too:
pack://application:,,,/ReferencedAssembly;component/ResourceFile.xaml
If you want to refer to the resource using the Referenced dll, you might use this. Remember the ReferencedAssembly refers to the name of the Assembly. Also remember if you place the ResourceFile.xaml in your root directory of referencedAssembly you should specify the path using component/ResourceFile.xaml. Therefore you can see, every file that is referenced through the ReferencedAssembly should always be relative to component folder. So for instance
pack://application:,,,/MyCustomdll;component/MyResource.xaml.
pack://application:,,,/ReferencedAssembly;component/subfolder/ResourceFile.xaml
Similar to one specified above, if the resource is placed within a subfolder within the assembly, you need to again reference that relative to component folder.
Source: http://www.abhisheksur.com/2010/04/pack-uri-to-reference-component.html