I don't think it's a true story. Usually, such characters and tales are created in the realm of imagination to capture people's interest and maybe teach some lessons.
The Sand Pebbles is mainly a creation of the author's imagination. It might draw inspiration from real-life elements but is not a factual account of an actual occurrence.
The story of 'Sand Pebbles' is a mix. While it may have roots in reality, it incorporates fictional elements to make it more engaging and cinematic. So, it's not a straightforward true story.
Bloodborne's story is set in a gothic horror world full of beasts and dark secrets. You play as a hunter trying to uncover the mysteries and survive the nightmarish encounters.