Guineas are attracted to the sight of blood, and very prone to cannibalism... they will typically continue to peck at a visible wound, making it worse, to the point of killing a bird, if it's allowed to get that far. There has to be a wound on one or all birds, so you might wanna take a better look. They typically do not fight at night in the dark, because they cannot see.
If it's a blood feather that is still bleeding or getting pecked at and starting to bleed again and the bird is shaking it's head... then that could account for blood splatters everywhere, so if you can find the blood feather again, then pull it out and clean up all the blood.
If you do find an open wound then it's best to clean it up and coat it well with a medication like Blu-Kote (stains everything purple on contact) and keep the reapplying the med until the wound is well healed. (Most feed stores that sell livestock meds will also have Blu-kote, in a bottle with a dauber, or in a spray can). You can also cage/crate the injured bird, to prevent the others from pecking at it/re-inuring it and just use something like plain neosporin ion the wound, but it's best to keep the injured bird near everybody else, but protected from further injury so that they do not reject the bird once it's healed and let back in with the flock.
I don't know if you have minks or weasels in your area, or how predator proof/secure your coop is, but it's possible it could be a night time predator attack. IMO... letting them out at night will most likely draw other predators in, to the smell of blood... so hopefully they make it thru the night!
Good luck, hope you get to the bottom of it and don't end up losing any birds.