How old are your chickens?
What type of food/treats do you feed?
Blood in the stool is often Coccidiosis. If I saw blood like that in my run, I would treat the whole flock with Corid.
You can find it in the cattle section of Tractor Supply.
Dosage is 1 1/2 teaspoons Corid powder or 2 teaspoons of 9.6% Corid liquid per gallon of water. Give for 5-7 days - make sure this is the ONLY water available during that time period. Mix a fresh batch at least once a day.
Do not give any extra vitamins that contain B1 (Thiamine) during the treatment period, but after treatment is finished, I would provide a poultry vitamins for a couple of days.
I agree that that is bright blood, and I would consider treating for coccidiosis with Corid. It also could have been fresh blood from a wound or blood feather. Check combs, wattles, legs and feet, for any cuts or bleeding feathers. I see an occasional bit of blood from some sort of chicken combat on my roosts, and sometimes never find the cause. If any chicken appears lethargic or hunched, get Corid right away. It won’t hurt them to use it. Are you seeing any other signs in a dropping?