Sounds to me as if you had a large, gross meat spot. A wild drake could have fertilized your duck, but the egg wouldnt' have developed without being at very warm temperatures for a while--close to 100 degrees for several days. Did anything chase your ducks? It sounds to me as if they were...
Nope. A blood spot is just a burst capillary as the egg is being laid. It is fine to eat. Blood spots will disappear over time and are the sign of a very fresh egg. Meat spots are fine to eat too, although many people remove them with the tip of a knife before cooking the egg.
Oh, and one more...