I tried to create a new Item Template for my Blazor Projects and it hasen't shown up in the "Add New Item"-dialog. The solution was to add TemplateGroupID, TemplateID, and AppliedTo in the TemplateData section of my .vstemplate file.
<VSTemplate Version="3.0.0" xmlns="http://schemas.microsoft.com/developer/vstemplate/2005" Type="Item">
<TemplateData>
...
<ProjectType>CSharp</ProjectType>
<TemplateGroupID>AspNetCore</TemplateGroupID>
<TemplateID>MyCustomTemplateID</TemplateID>
<AppliesTo>DotNetCoreRazor</AppliesTo>
...
</TemplateData>
<TemplateContent>
...
</TemplateContent>
</VSTemplate>