If they walk approx. 5 hours north, they will find a larger cherry blossom tree with a door. There's no signs on the door - or any other implications what might be behind it.

Behind the giant cherry blossom tree is the barrier than leads to The Darkness. Touching that will feel cold and unpleasant, but nothing else happens. They can see a cherry blossom petal fall through the barrier and evaporate into dark smoke.