Are you sure it's blood? I used to own rats, all deceased now unfortunately. Sometimes a reddish, blood/rust colored liquid discharge will be present around the mouth, nose, or eyes. Occasionally when the rats breathe, this liquid will kind of spray, possible resulting in the spray patterns you've noticed. If you haven't really noticed any wounds, this may be the case with your rats. The stuff isn't always visible on the rodent itself. When my rat Jasper started to get sick, I noticed what I first perceived as blood on his white bedding. A little research told me otherwise.
Check to see that your rats are healthy, as such discharge may be a sign of respiratory illness. Keep an eye out for sluggish rats, gurgling noises while breathing, residue around noses, etc.
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