In this blog post you can find some useful tips about getting started on creating a new project or item template in
Expression Blend.
Source: Unni's Blog
Expression Blend 4 has support for authoring custom project and item templates, similar to Visual Studio. However, our support is a strict subset of what Visual Studio supports, and a few changes are necessary to the .vstemplate file to have the projects and items correctly displayed in Blend. A notable exclusion to what Blend supports, when compared to Visual Studio, is the ability to invoke custom wizards that can be used to customize a template when a new template is instantiated.