What’s going on?
Peer into the blue light, and you'll see little dots or squiggles made by white blood cells moving through capillaries—the narrowest blood vessels—at the back of your eye. White cells let blue light pass through them, creating the moving dots; red cells block blue light, creating shadows that follow the dots.
Blood cells are pushed along by pumping of your heart. So if your heart speeds up, the rhythm of the dots (cells) speeds up too.
Although these cells are always there, you ordinarily don’t see them unless you’re gazing at a deep blue sky.
White blood cells are barely big enough to move through a capillary, while red cells are smaller. So a traffic jam of red cells forms behind each white cell. That's why you see a bright dot followed by a darker shadow.