Glad you aren't finding any more blood.
21weeks is fairly young to start laying eggs unless they are all production breeds. So, I would say it's normal.
Did you look at their vents?
That look like blood on poop.
How long have you had the pullets?
I would watch their behavior, see if any of them act lethargic, puffed up, not wanting to eat/drink.
Look at overnight droppings to see if there's more blood in the poop.
To me it's not normal.
It...