Dark Souls 3 links its lore to Bloodborne’s with a very simple thing: Blood.
FromSoftware is meticulous about lore details to a degree bordering on absurd the connections between the games have been there all along, but the Dark Souls 3 DLC, Ashes of Ariandel, brings them into the light. But there are too many signs in Ashes of Ariandel to deny that Dark Souls may be a prequel to Bloodborne.