Blood is a special fluid' is a quote from Faust (by Goethe), and the title of a lecture by Rudolf Steiner in the period 1906-10 to 1907-01.
It points to the fact that blood is not just a physical bodily fluid, but an important spiritual substance as it relates to and is the carrier of the 'I' bodily principle of Man. The mineral substance analyzed after removing blood from the human organism, is no indicator for the functions of spiritual fluid in the body of Man.
https://anthroposophy.eu/Blood_is_a_special_fluid#