Although we rarely find blood spots in eggs from supermarkets today, the custom to check every egg is still entrenched in observant Ashkenazi homes.
The Talmud (Chullin 64b) prohibits eating an egg with a blood spot if it might have been fertilised. But most chickens in industrial farms never see a rooster. Any blood spots in their eggs are from tissue irregularities. The Shulchan Aruch rules that after removing such a blood spot, you may eat the rest of the egg.
In England, many Jews throw out an egg rather than removing the blood spot. However, as the rest of the egg is perfectly kosher, one should be sensitive to the problem of wasting food.
A blood spot in an organic or free-range egg might indicate fertilisation (and therefore be prohibited) depending on the conditions at the farm of origin. The return of some farms to more humane farming methods - a good thing - makes it important to be careful about checking eggs from those sources.