Are you thinking of "bloody murders" (common in crime fiction or horror) or perhaps "bloody Marys" (the cocktail)?
In British slang, "bloody" is often used as an intensive. "Matures" could refer to a specific group (e.g., "Those bloody matures are at it again"), though this is not a standard idiom. bloody matures
This may be a fragment from a specific conversation, a niche piece of media, or a slight misremembering of a similar phrase. Here are a few possibilities to help you find what you're looking for: Are you thinking of "bloody murders" (common in