Rockstar has a habit of purposefully making trailers inaccurate for the sake of story spoilers. If you look at the GTA 5 trailers, quite a few cutscenes are the same but take place in a completely different place. I think a lot of the Blackwater stuff you see in trailers is just that, hiding spoilers. Also if Rockstar wanted to show off Blackwater in a trailer they would have to use Arthur Morgan. Because they also don't want to spoil John being playable.

So no, I don't think an entire chapter of the game with Arthur in Blackwater was cut. But it is obvious that Arthur was at some point meant to explore Blackwater and New Austin during free roam. But Rockstar changed their minds on that.

@Rvb forever I agree with what you mean on the trailers that is very true, But it's the journal that gets me. If Rockstar didn't plan on a Blackwater chapter then I don't think they'd make a whole journal chapter.

I agree. The Blackwater events in Arthur’s journal are very vague in comparison to the events in other chapters, so it clearly wasn’t supposed to be part of gameplay. The Blackwater events (and prior to that, such as in the Grizzlies) are merely there for insight - not cut gameplay. If it was cut gameplay, then Mac, Davey and Jenny would’ve had a role. If som they would have been written about, voiced, and Rockstar would’ve released biographies of them and possibly other information on top of that.

Also, as Rvb said, they wouldn’t spoil Arthur’s death by only showing John in Blackwater. It was, in effect, a teaser trailer.

On another note, you are right about the image of Arthur and John (the one to represent Epilogue Part 1) being from the mission “Red Dead Redemption”, and specifically from Beaver Hollow.