The Surface TouchPack is a collection of end-user applications for a multi-touch computer. There is no SDK associated with Surface TouchPack. I have heard of people linking against assemblies distributed with the TouchPack, but that is technically against
the license and you cannot redistribute those assemblies.
Blake.NUI is designed to be used in multi-touch NUI applications in general but especially to augment Surface SDK and Surface Toolkit. (Surface SDK targets the Microsoft Surface device while Surface Toolkit targets Windows 7 computers.)
For example, the core Blake.NUI project does not depend upon either Surface SDK or Surface Toolkit. It could be used in any WPF application, including ones that use provide custom multi-touch input to WPF 4 or Win7. The Blake.NUI.Surface and Blake.NUI.SurfaceToolkit
provide a few extensions for the core Blake.NUI project that allow you to use it with those respective SDKs.
Blake.NUI does not replace the role of Surface SDK/Toolkit but focuses on going beyond what it provides and helping you create better NUIs in less time.