In the US supermarket grade A eggs are candled so the chances of getting a blood spot are miniscule. However, if one buys direct from a farmers market or organic farm one has no assurance that she is not buying a fertilzed egg.

Aryeh Shore

Also, most of the eggs sold in supermarkets come from what we in the UK call "Battery Hens" whose eggs are never fertilised.

Aryeh,
Thanks, what about the 3 egg minimum? You didn't address that question.

I thought someone else answered that one.
The concept is bitul brov. That is less than fifty one percent of one substance make the other substance non-existant. This is considered bitul from the torah.
The classic example is camel hair and sheep wool. If most of the raw material is mixed togeter and the majority is Camel hair, the resultant material is considered camel hair and can be mixed with flax although flax and wool together are forbidden (Shatnaz).
In the case of food, the Rabbis require bitul of one in 60 since a food can be tasted in very small quantities. In addition, objects which are clear entities like an egg, may not have any bitul unless they are scrambled. However, in this case, since we aren't talking about a real issur, just something so the actually law will not be forgotten, then bitul brov is sufficient, i.e., if three eggs are cooked and one is found to have a blood spot, the other two eggs are O.K. and the pot does not need to be kashered.
The blood spot in an unfertilzed egg is caused by a capallary bleed when the ovum separates from the ovary which looks like a bunch of yellow grapes so it called אשכול in Hebrew.

Aryeh Shore