Sometimes a blood vessel can burst during laying which can cause some blood, but it's usually not a lot. There may be a smear on the egg shell, or a bit on nesting material. If you see bloody droppings again, a picture might help. I would definitely keep an eye on them, make sure no one starts to look off at all. New layers can have glitches while their bodies figure things out, so some of those things may work themselves out with a bit of time. If you continue to have thin shelled eggs, then I would try to identify which birds are doing that, they may need more calcium if it continues. Look at what you are feeding, too many treats can be a problem, too many of some kinds of greens can interfere with calcium absorption. Make sure you have oyster shell available in a separate feeder all the time. Needs can vary bird to bird and season to season, some birds need more than others, and having it available lets those that need it take what they need.