The bluestone is a boulder. I found a little story about it in the book Gränichen 1950-2008, Our Village in Pictures.
At the "Buckhalde" there was or still is a large conglomerate rock. It doesn't take much imagination to imagine the remains of a castle ruin. Buckhalde is "Gränicher German". In written language the hill is called "Burghalde".
The story goes that the last lords of this former castle were evil robber barons and murderers. Because of their misdeeds they did not find eternal rest and so night after night at 12 o'clock they had to climb out of the ground at the castle stone and drive around in a fiery carriage with horses of the same kind on the paths of their crimes. Old people would have seen the glowing vehicle galloping through the barn in the field wagoner's farmhouse, which was then a detached one. The two threshing floors had to be left completely open at night so that the thugs could get through as quickly as possible and the house didn't catch fire. The ghost ride ended at 1 a.m. at the "blue stone", a boulder from the glacial period. There the hellish team, including the wagon and band of robbers, disappeared back into the ground with a hiss and thunder.
And now the highlight of the story, and it is true and can be verified at any time.
Anyone who hits their head hard against the blue stone can still see the robber barons in a fiery shower of stars, at any time of day. Depending on the strength of the impact, the apparition becomes more or less clear.
Try it out and see if it's actually true when you pass the "blue stone".
PS: Bikers must of course take off their helmets first.