I'm trying to add outlines to my text, from the docs I saw something about noesisguiextensions:

xmlns:noesis="clr-namespace:NoesisGUIExtensions"

Which I added to my xaml project and I attempted to add it to a textblock, but I get an error about it not being referenced in Unity so my question is, wher can I get the C# DLL to use the extension in blend and Unity?

This is the project:

https://github.com/Noesis/noesisgui-contrib

where all our extensions to Blend are published, for now there is only one, the stroked text.

So to use these in Unity I just include the dll reference in the blend project? Where should I place the dll to be referenced?

It is a .cs, simply add it to your blend project and compile. As you can see, there is no implementation at all, it is just a dummy class that avoid errors when compiling in Blend, but you won't see stroked text inside Blend.