According to Eat or Toss, eggs can have blood spots in the yolk or in the white.
MedicineNet says both result from small blood vessels breaking in the hen's ovaries or oviduct.
The little imperfections that end up floating in the egg white indicate bleeding after the egg discharges from the follicle, unlike spots in the egg yolk.
When it's in the egg white, it's a meat spot (via Eat or Toss).Brown-shelled eggs have more spots than white-shelled eggs.
According to Flock Journey, 18% of brown eggs and 0.5% of white eggs contain meat.
A meat spot in an egg indicates freshness, according to Flock Journey. The spots fade and become almost invisible as eggs age.