Blazor uses webAssembly (WASM) for client application rather than JavaScript. For server side app communication it uses SignalR .
It uses C# to build interactive webUI components. Both Client and server side code is built using c# and hence can be shared.
It can be used in all scenarios you would use AngularJS , ReactJS , Vue.JS etc.
You can develop webapp , clientapp and even desktop apps*. As a future scope you can build mobile native apps using blazor and xamarin.