@Pickels correctly points out that Flux is usually an interface architecture. It was originally conceived only for the front end.
However, it has been adapted to the internal architecture by several people, especially people from Yahoo. But Flux doesn't have to be the back architecture for serving an isomorphic application with React.
I noticed two different approaches to how React + Flux can be served isomorphically. One of them is the Yahoo approach, and the fluxable infrastructure and their examples are a good place to start looking at this. This is Node, as you said.
However, if you look at how Soundcloud approached the problem , you will see something completely different. Their rear end is in Scala. But they deploy the Nashorn instance and submit one initialization action through the Flux application in that instance to get the initial render of the React view layer. Then they send it as HTML.
If you are looking for Flux-like stuff in the .NET community, you might want to check out the framework of the CQRS- based application (Responsibility for responding to a command request)), which I believe has been popular in the .NET community for some time. Flux development was influenced by CQRS, data flow programming, and reactive programming, and CQRS is probably the closest Flux cousin in the family tree of architectural models.