When to use BlockingCollection and when ConcurrentBag instead of List?

You can indeed use a BlockingCollection, but there is absolutely no point in doing so.

First off, note that BlockingCollection is a wrapper around a collection that implements IProducerConsumerCollection<T>. Any type that implements that interface can be used as the underlying storage:

When you create a BlockingCollection<T> object, you can specify not
only the bounded capacity but also the type of collection to use. For
example, you could specify a ConcurrentQueue<T> object for first in,
first out (FIFO) behavior, or a ConcurrentStack<T> object for last
in,first out (LIFO) behavior. You can use any collection class that
implements the IProducerConsumerCollection<T> interface. The default
collection type for BlockingCollection<T> is ConcurrentQueue<T>.

This includes ConcurrentBag<T>, which means you can have a blocking concurrent bag. So what’s the difference between a plain IProducerConsumerCollection<T> and a blocking collection? The documentation of BlockingCollection says (emphasis mine):

BlockingCollection<T> is used as a wrapper for an
IProducerConsumerCollection<T> instance, allowing removal attempts
from the collection to block until data is available to be removed.
Similarly, a BlockingCollection<T> can be created to enforce an
upper-bound on the number of data elements allowed in the
IProducerConsumerCollection<T> […]

Since in the linked question there is no need to do either of these things, using BlockingCollection simply adds a layer of functionality that goes unused.