That's either called a meat spot or blood spot depending on location inside the egg, yolk or white. I can't keep them separate in my mind.
We never see them in commercial eggs because their candle process is much better than ours. They simply send those to be used in commercial applications like school lunches or where they eggs are inside a processed food. If too much then pet food.
They are completely safe to eat even if not appetizing to see. If you google meat or blood spot in eggs you will get several sites with the same basic info as what I gave. If I don't feel like eating them I feed them back to the pets.
Also doesn't mean your hen is ill.
I always explain about them to anybody who isn't familiar with farm fresh eggs so they don't panic and think there's a chick in there or some other abnormal grossness.