Incorrect. Otherwise, how would Obi-won know he could block it? How would Yoda know how to manipulate it? Did you think they were practicing against Sith all this time?
"I think in ROTJ they deliberatley tried to play on the ambiguity of Lukes character, hence the basic black outfit and cloak (like father, like son) , hinting that he could possibly go either way, remain good or join his father on the darkside"
Exactly. There is always the risk that Luke will follow in the footsteps of his father. In fact, for a few seconds in the duel, he almost does.
Remember, folks. Luke wasn't a Jedi until the end of ROTJ. That's when he finally understand the nature of the Force, as well as its lure.