In a lot of cases, yes. Originally hatcheries most likely had real Ameraucanas (not sure on Araucanas though) but that doesn't mean all EEs have them somewhere in their genes. Some are so crossbred that there is no blue gene left (hence the brown egg) and no pea comb or even similar body type, and others never had Ameraucana in their genes in the first place. In general though, yes - Most hatchery EEs originate to real Ameraucanas, years and years ago.
From all that I have read and understand there were orginally two types of blue egg ;laying breeds imported from South America (Chile). I believe that the current Easter Eggers, Ameraucana, and Araucana were all developed from these two types.
That's one reason that it really gets me that people are calling Easter Eggers mutts and mongrels. They are also calling their own mutts ans mongrels. One thing I can say about the Easter Eggers though every one that I have ever owned have been the greatest egg layers.
The Cream legbar does have some of this blood in them. According to The Autosexing website they have the ameraucana blood in them.
Never heard of a blue egg laying Dorking. They would just about have to have some somewhere I would think. Is there another country that have had green or blue egg laying chickens?
If you want to learn more go to the Ameraucana breeders club's site, Ameraucana.org and the Araucana club's site, Araucana.net. You will find detailed histories of the breeds there as well as a lot of other fascinating information.