Unfortunately, after a ton of research and testing, when working with client smart cards the server does not and will not have access to the user smartcard certificate to utilize it to make calls from the server to the server, even on behalf of the client.

The problem cannot be resolved as a server side only solution. The client browser must make the appropriate API calls and supply the credentials to authenticate/authorize with the endpoint API.

BLUF, at this time, this is not possible using Blazor server-side application that has endpoints that require end user smart card certificate authentication.