Wordpress RSS Parser
The Rss helper functions are inside the parse function, even though they don't use any of the state inside that scope. Consider moving these up to the module level, even though they are not used anywhere else. By doing this the main function is shorter and easier to follow. You could also make those functions private when you pull them out.
Seq.cast can throw an exception. Even though you know that every item will be an XmlNode in this case, it can be safer to filter out objects that are not of the required type while casting. Unfortunately, F# doesn't have a function for this in the Seq module but you can always use the one already in .NET: channelNode.ChildNodes |&gt; System.Linq.Enumerable.OfType&lt;XmlNode&gt;

You fetch the RSS feed in the constructor of BlogController. This will happen once for each request. Using Async.RunSynchronously will block a thread, so you lose the benefit of using Async in the first place. You should avoid Async.RunSynchronously whenever you can. You can pull out the RSS fetching and parsing into another function that returns an async. Call this function from each controller method, do more async processing if necessary and then use Async.StartAsTask to return a task from the method. ASP.NET will then be able to run your code without unnecessary blocking.


EDIT:

I'm not too familiar with ASP.NET MVC, but based on this issue I think something like this should work:

let fetchRss() = async {
    let! content = fetchUrl "https://christopherjmcclellan.wordpress.com/feed/" 
    return Rss.parse content }

type BlogController() =
    inherit Controller()

    member this.Index() = async {
        let! rss = fetchRss()
        return this.View(rss.Channel) }