If I was going to say Bloodborne was about one thing (which it's not), that one thing would be exploitation.

It's about the exploitation of women, of men, of the working class, of the sick and disabled, of colonies, of history and the past, of love, of life, of dubious gifts given by the gods. All the name of knowledge, power, and evolution.

Pretty much every story thread in the game touches on either exploitation directly, or on the brutally violent cycle of beasts and hunts and blood ministration that keeps Yharnam's common folk paranoid and in line. The whole game is a running commentary on oppression and exploitation, from both the perspectives of the oppressed and the oppressors!

I could probably write a dissertation on looking at the game through this lens and picking apart what it has to say if I had more time and energy. I love how much Bloodborne is in dialogue with the racist and imperialist attitudes found in the Lovecraft and Victorian culture that inspired it, and how it examines and refutes those attitudes, as well as what it has to say on more modern forms of exploitation and what it means to live in the imperial core.