as Blazor server is an asp.net core signal/r app not sure why you want to reverse proxy. But you must configure IIS reverse proxy for single/r.
https://stackoverflow.com/questions/41566472/signalr-with-iis-10-and-arr-3-0
also special care must be taken if you you configure the reverse proxy as a subsite (/example). the easiest if the site you are reverse proofing to uses the same subsite. be sure all html links are relative.
we could help more if you gave more info:
- the Blazor server site url (I assume its another computer or it would be hosted directly)
- the IIS reverse proxy site url for the Blazor app
- the reverse proxy settings.