In this English exercise, they say
The girl had cut herself with a knife and blood was coming out of her thumb.
Why not "her blood was coming out"?
"I cut myself and blood came out" or "I cut myself and the / my blood came out". Right at that moment, you saw the specific blood came out.
Also, Why do native speakers say "I slapped the mosquito and blood came out", which is confusing because we don't know whose blood it is "me or the mosquito"?
Suppose that the mosquito sucked someone's blood and when it was flatten or killed by me, the blood in the mosquito came out, not my own blood. Would we say "I slapped the mosquito and its blood came out"?