Why chicken eggs sometimes contain blood?

To a certain extent you can get some red dots in chicken eggs because they've been fertilised, but it's perfectly possible to get red dots and a little bit of blood if they're not fertilised. That's because the egg gets made in the chicken's oviduct. It descends the oviduct and has the shell made around the outside. Sometimes as it's going down the oviduct, a blood vessel in the wall of that oviduct in the mother chicken can burst and spill out and get into the material that's being laid down in the egg. This means that you have a little blood spot in the egg that then gets wrapped up in the shell. Most egg producers shine lights through the eggs in order to see if that's happened and weed them out. Pale eggs are easier to weed out than the brown ones, which is why you tend to see it more often in brown eggs. It's not in any way pathological having the spot in them and you can eat those. They're harmless.