Microsoft Entra External ID
Introduction
First, a confession. I have previously used Azure B2C, the predecessor to Entra External ID. However, Azure B2C is no longer available for new customers, although existing tenants can continue to use it and it will continue to be supported, probably until 2030.
In many ways Entra External ID is the obvious security provider to use with a Blazor application. Both Blazor and Entra External ID are Microsoft products and one would hope their integration will be straightforward.
Cost
Entra External ID is free for the first 50,000 MAU for email authentication. SMS and voice Multifactor Authentication (MFA) are billed per attempt.