Migrating fails without any error
A few things I see. const JOIN = 'n.vid = nr.vid'; this is dead code. Not sure where it comes from but as far as I am aware there's nothing in DrupalSqlBase or SqlBase that uses the constant JOIN. I am not sure what this even wants to be.

Your process copies the type and then in the destination I think you try to set the type/bundle to article but that's simply not how this works. Everything under the destination key is passed to the destination plugin as configuration and the way we designed and implemented this system the destination plugins do not change a thing, they just import.

So:

source:
  plugin: progweb_custom_blog
  constants:
    type: article
process:
    type: constants/type


In your particular case, you could use a map similar to und: en to map blog to article but if you want article nodes then you must use either the constants or the map.