Don't mix mainstream adoption at the same level as regular JavaScrip and Typescript, with availability.
Microsoft would wish Blazor would take off like React and Angular, in reality it is seldom used outside .NET shops intranets in a way similar to WebForms.
> Blazor is seldom used outside .NET shops intranets
So, in other words, widely used in lots and lots of deployments.
Do you have a number for us?