In Thus Spake Zarathustra, Chapter VII says this:
Of all that is written, I love only what a person hath written with his blood. Write with blood, and thou wilt find that blood is spirit. It is no easy task to understand unfamiliar blood; I hate the reading idlers. He who knoweth the reader, doeth nothing more for the reader. Another century of readers--and spirit itself will stink.
My guess is that if one writes with his own blood, he must be writing a message worthy of his own vital fluid (blood). It must be an important message. It will be draining to do so, and he will lose his spirit. Is that what Nietzsche meant by the first two sentences? Is unfamiliar blood a metaphor for passionate discourse from someone you do not know? Is that what Zarathustra is saying? What does the last sentence mean about the spirit sinking?