There have been theories, some more plausible than others, and they're certainly fun to bat around. But Dark Souls loremaster VaatiVidya has has his definitive say on what the hell Bloodborne is about. If you've got 30 mins, it'll open your eyes.
Vaati remains dutifully academic in his exploration of Yharnam lore, and when he departs into the realm of speculation, he'll let you know. He's also sourcing his points mid-video now, in case you were curious about where all the info comes from.
For the most part, it's inarguable. But it doesn't necessarily contradict some of the more speculative fan theories that have been thrown around.
Bloodborne is highly polished but smaller than any Souls game, so I was worried that it might not be able to support this type of content. But this 30 minute video chugs along without any filler, which is great to see.
Vaati covers the history of the church, the city, the dream, insight vs blood, and more. As for the endings, that'll be another (large) video later on.