You will occasionally see blood on the outside of an egg - any minor irritation to the inside of the vent can result in a bloody shell exterior as it is an area of the body rich in vessels that is stretched significantly as the egg passes through. I wouldn't worry too much until/unless it becomes an even more frequent thing - if you are concerned, and if your birds are manageable, you can always catch them up and give them a once over to rule out any obvious signs of trauma in the area.
I did notice some blood on an egg - so I started inspecting vents, and found that Elfie - who does not like to swim - had some dried poo on the feathers around her vent, and I think that caused extra pulling when she laid the egg. I trimmed it off - carefully!
I just read on another thread that was discussing this same issue with chickens that the cold weather can make them tense and that the vent needs heat to dilate. The extra tenseness of being cold can cause those blood vessels to be irritated as the egg passes through the vent.