An phrasal attributive referring to violence and uproar, especially in a written story. Applied to adventurous and melodramatic stories, which may include sword fights and, apparently, big storms among their many attractions. (Okay, so the thunder part probably isn't meant literally).
It almost certainly comes from the oath "Blood and thunder!", a general exclamation used to show that one is angry. The first recorded occurrence of the this phrase being used as an adjective was in 1852, and it was actually "blood, thunder, and whiskey", referring to over-the-top newspaper articles. By the late 1800s the phrase was so common that it was often shortened to simply 'blood' -- "blood stories" and "blood books". The Penny Dreadfuls were sometimes referred to "penny bloods". These days the entire phrase is used; if you don't no one will know what you're talking about.