Question

Why blood is a mixture and graphite is an element?

Answer 1

Blood is composed of water, proteins,  and so on. I.e Mixture of everything.

Graphite is made up of only carbon. So it is an element.

Answer 2

blood is a mixture because the blood is composed of proteins, RBCs , WBCs , fats etc
it has many minerals mixed in it

graphite is not element , its an allotrope (each of two or more different physical forms in which an element can exist) of carbon (which is an element) so blood is mixture and graphite is an allotrope
hope this helps