From the official documentation, in order to get started with Blazor, you have to install the latest preview of VS 2019 with the ASP.NET and web development workload along with the Blazor extension.
Also, In the Create a new ASP.NET Core Web Application dialog, confirm that .NET Core and ASP.NET Core 3.0 are selected.
Refer to this getting started link for more information about Blazor prerequisites.
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