I guess it would be Wolfenstein 3-D but at the time I didn't even notice the blood at the time or really find it that violent really. But this is all likely because I was very young at the time (5) and it was my first video game so I guess I really didn't have anything else to hold it up to before that? Whatever the reason it just wasn't a game that came across as grim or "mature"
For me, I still count my first "mature" game as Re-Loaded for the Playstation. It was the first game my dad and I rented when I got my PS1(I was 8) and I was already blown away by the fact that the CD had blood and bullet holes on it, But that was just a small taste of what the game had in store.
Soooooo much blood, Women with people hanging from meat hooks, A giant man with part of a Bear Trap for his lower jaw ect. Yeah, innocence was lost that day, and I just kept pressing through to gorier and stranger games heh.
EDI: heck now that I think about it there was Mortal Kombat and Killer Instinct before Re-Loaded but again for some reason those games just didn?t affect me like Re-Loaded and the later games that I discovered for Playstation (Resident Evil, Nightmare Creatures, Tenchu, Unholy War, Abes Oddysee ect)