An idiomatic expression meaning extremely painstaking or grueling effort—working so hard it figuratively makes blood seep.
彼は合格のために血のにじむような努力を続けた。
He continued making painstaking effort to pass (the exam).
その研究は血のにじむような苦労の末に結実した。
That research finally bore fruit after grueling toil.
血のにじむような練習を重ねた選手ほど勝負に強い。
Athletes who have trained with grueling effort tend to be strong in competition.