If you see black spots inside of your hard boiled egg (white part or the yolk) it is most like a blood-spot/meat-protein, which is now cooked & solidified.
When heat is applied to the egg as it boils, the sulphur links break down and can start to change into hydrogen sulphide, which then reacts with the iron in the egg yolk to produce iron sulphide. That iron sulphide is what creates the odd color around the yolk.
Is it safe to eat a Black Egg Yolk? Hard-boiled egg yolks that get a black shade after cooking are edible, but when a raw yolk has black spots, it's not safe to eat. The egg is spoiled and can contain bacteria like Salmonella or E. Coli that can cause food poisoning.
Eggs with dark spots
It's completely normal to see brown or red spots floating in the egg white or on the yolk; these are known as blood spots. Dark spots that appear directly on the underside of the egg shell, however, are a sign of mold. If you suspect that a dark spot is mold, discard the egg.
A greenish-gray ring may appear around a hard-cooked egg yolk. It's unattractive, but not harmful. The ring is caused by a chemical reaction involving sulfur (from the egg white) and iron (from the egg yolk), which naturally react to form ferrous sulfide at the surface of the yolk.
Simply fill a bowl with cold tap water and place your eggs in it. If they sink to the bottom and lay flat on one side, they are fresh and good to eat. A bad egg will float because of the large air cell that forms at its base. Any floating eggs should be thrown out.
The best way to test if your eggs are still good after you've boiled them is, again, the smell test. Basically, you'll notice the smell is off if an egg is starting to go bad. This is true for raw eggs and it's true for boiled eggs—the funk doesn't lie.
The most accurate way of testing whether your eggs have gone bad is to crack them open. If the yolk or white has any discoloration, the egg is no good. Discoloration is an indication of bacterial growth. If your eggs look good but have an off or sulfuric smell to them, they are bad.
A slimy or chalky shell can indicate bacteria or mold growth, while a brown shell is a sign of spoilage. If any of these signs are present, toss the egg to be safe. And as stated, a foul odor is also a sign of rottenness, so you should discard the hard-boiled egg to avoid getting sick.
Overcooked hard-boiled eggs are safe to eat (in moderation) as they release a minimal amount of iron sulfide, which would be dangerous if consumed in excess. If an egg is overcooked, it'll develop a bluish-green tint around the yolk, but as long as it is fresh, it can be eaten safely.
The best way to determine if your egg is spoiled is by cracking it open into a bowl. If the egg white is pink or iridescent this is an indication of spoilage due to Pseudomonas bacteria. Some of these bacteria can make us sick when eaten and they will produce a greenish, fluorescent, water-soluble color.
In a properly hard-boiled egg the white should be fully set but not rubbery, while the yolk should be firm and bright yellow. If the yolk has a green ring around it, that means you've overcooked the egg. The green tinge results from a chemical reaction between the sulfur in the egg white and the hydrogen in the yolk.
However, when consumed raw, an egg infected with the pathogenic bacteria salmonella can cause serious illness, particularly to the immuno-compromised. Unfortunately, salmonella is odorless, tasteless, and displays no visual cues when present.
Fungal rotting:
It is the final stage of spoilage by mold. In this case, mycelium of the mold grows through the pores and cracks in the shell. Jellying of egg white may occur and colored spots may be produced. Hypha of mold grows through the yolk membrane and rupture it, so that yolk mixes with the white.
These dark splotches are created early in the process as the egg is traveling through the hen. As the egg passes through the oviduct (the organ that processes the yolk and adds the shell), it spins. If the egg spins too slowly as it moves along the tube, it can end up looking a little speckled.
In short, you can eat eggs two months out of date. It'll be fine. If you eat an outright bad egg, that's a different story. Eggs pretty exclusively spoil from contamination, and the signs and symptoms are pretty clear if you consume an egg that's been harboring salmonella.
Salmonella illness can be serious and is more dangerous for some groups of people. Most people who get sick from Salmonella have diarrhea, fever, vomiting, and stomach cramps. Symptoms usually begin 6 hours to 6 days after infection and last 4 to 7 days. Most people recover without antibiotic treatment.
Shelly McKee, a food scientist at Deb-El Foods and an expert on egg chemistry, said that boiling an egg for a long time would cause it to expel moisture. As the proteins in the egg white clump together ever more tightly, they would squeeze out the water contained within the egg.
Don't add the eggs until the water reaches a steady boil. Add salt to the water before adding the eggs. if you boil the eggs longer than 12 minutes, the yolks will turn grey around the edges.
4 minutes for slightly set yolk and set white. 5 minutes for a medium cooked firmer yolk and white. 6 minutes for hard boiled with lightly soft yolk. 8 minutes for firmly hard boiled.
Hard-cooked eggs can also have black or brown spots on the albumen (egg white) from overcooking or burning. Discoloration due to overcooking does not impact food safety and these eggs are safe to eat.
According to the Food and Drug Administration, you can keep hard-cooked eggs in the refrigerator for up to seven days after they have been cooked. 1 (Here's the best way to make them.) And it doesn't matter whether the eggs are already peeled or still in the shell.