Locate a cliff near the waterfall north of MacFarlane's ranch; it will say "Tall Trees" once you get to the edge. Get arrested at this spot, and you will be transported to Blackwater. To get back to New Austin, quick save the game, and then kill yourself. Note: If you buy the Manzanita Post property, you will have to use your horse to jump the unfinished bridge north of Thieves' Landing to return.