According to the summary of “Young Goodman Brown,” Goodman Brown witnesses “an evil ceremony that implicates his wife, Faith, in sin [and] he...

Yes, I believe that Goodman Brown returns from the ceremony with darkness in his soul. 

The question is, as you have indicated, does he have the darkness in his soul before that time?

I think that you can see Brown's vulnerability to darkness in the first paragraphs of the story. To be specific, when Faith warns him not to go and Goodman insists on going anyway (paragraphs 2 and 3), that's a warning. Look at the characters' names. This is a case when a good man is ignoring the warnings of his faith. That's not good in any situation and, in a story about an explicitly Christian community, it's a clear symbol of impending fall (sin) due to pride.

This leads directly to an answer to your last question: yes, Goodman Brown definitely deserves blame for what happens. I'd say he deserves quite a bit of blame. Again, start with the story's opening: he refuses to listen to his faith. That's blameworthy in itself. If you include the fact that he (Goodman) rejects his wife Faith's concerns, he deserves even more blame.

This pride is underscored when he says, "What if the devil himself should be at my very elbow!" and then continues to walk the path. This is a man who decides to leave faith and go on expecting to encounter the devil. Since no man (in a Christian frame) can resist the devil without faith, he deserves blame.

As for when he becomes bitter, I'd argue that this change happens only in the last few paragraphs of the story. Before that, Goodman is wrapped in his arrogance. It isn't until that is punctured that the bitterness sets in.