Sony hasn't publicly declared a release date for Bloodborne on PS4 just yet, but information posted to the game's listing on the PS Store indicate spring 2015 with a more specific March 31 release date. It's possible that could be a placeholder as that date marks the end of Q4, which is what's mentioned in Sony's official E3 brochure, but there's some more evidence pointing towards March 31 as the official date.
For starters, when visiting the Sony PlayStation Store from a web browser you can see the "anticipated release date" of March 31, 2015. Now normally an end-of-month date usually indicates a placeholder, but in this case it's a Tuesday which means it could be correct.
The real catch is when you visit the PlayStation Store from you PS4. As we reported last week, when looking the game up on our PS4 (screenshot above) we noticed that pre-loading starts on March 26, 2015 with the ability to play opening on March 31, 2015. Again, these could be mere placeholders, but I have a hard time believing Sony would post a placeholder for pre-load functionality.
So to recap: release date falls within the rumored Q4/spring release window AND on a Tuesday. Two separate PlayStation sources indicate a March 31 release with one giving a pre-load date as well. We do know one thing for certain, however, and that is Bloodborne will be released in 2015 barring any delays.