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.