Testing a list of proxies for validity concurrently
You are starting two Task in ValidateProxiesAsync - one in the method and another in the extension method.  Really seems it should be one Task.  

Also judges is an IEnumerable that you are constantly doing ElementAt.  Should either do First and store that value for each call or pass in an IList or Array is that's what it really is if you need to access it by element 

As for Parallel.ForEach vs TPL DataFlow I think that depends on if you are building a data flow mesh (TPL DataFlow) or just want to process items in Parallel only (Parallel.ForEach)