As static web apps allows to make web apps with so many javascript libraries, does it support Blazor under the hood or it's not supported as of yet?
We are working on this support! Today you can use client-side Blazor support as it compiles into static assets.
Here's an article on how to do it today by modifying the Actions: https://www.timheuer.com/blog/hosting-blazor-in-azure-static-web-apps/
Thanks for sharing the article Aaron.
Not sure if this was possible when you originally asked this question, but Today you can also do this without modifying the Actions.
I think the trick is to configure the right paths when create the static web app:

It works out of the box, check out my how-to: https://swimburger.net/blog/dotnet/how-to-deploy-aspnet-blazor-webassembly-to-azure-static-web-apps
Announced at Ignite today: first class support for Blazor (and C# Functions) for Static Web Apps. Interesting links:
Most helpful comment
Announced at Ignite today: first class support for Blazor (and C# Functions) for Static Web Apps. Interesting links: