When I googled this awhile back, what I found was that this happens with fertilized and non-fertilized eggs, and it is normal. There is nothing that can be done. I think I read one BYC'er say if I remember correctly that she thought it happened more with certain hens in her flock.
You might do a BYC search too to get more info.
I have seen blood spots in my eggs on occasion. I simply remove them before I cook the egg or whip the batter.