If everyone looks fine and you are not seeing any other blood than I wouldn't worry too much yet, but have a good look around, are they molting, is one being pecked, there's a reason why someone is sleeping in the nestboxes, you could wait until dusk to find out who it is and see if you can see any injuries or reason behind the blood drop and the behavior.
How old are your chickens? As previously said seeing that once is not alarming, but there looks to be a pinkish tinge to some of the othere droppings, at least from the picture. In younger birds blood in stools can be coccidiosis, but in older birds blood could be capillary worms. Cocci is treated with Corid in the water, and capillary worms are treated with fenbendazole 1/4 ml per pound of weight given orally for 5 days in a row. A vet could also test several of the droppings for both of those.
I get that on occasion because my rooster is a klutz with a big comb and bumps it on things just a few drops here and there, did one of your ladies maybe get a nick on their comb or feet? As others have said it may be parasites or just some picking.