Note to JULIUS CAESAR, 4.3.115, 'blood ill-temper'd'
Cassius means that his bad humor is not really his fault. According to the common thought of Shakespeare's time, health of body and mind depended on a balance of the four essential "humours" of the bodyphlegm, yellow bile, black bile, and blood. When those humours get out of balance the person is "ill-tempered," unbalanced. (Our terms "bad humor," "good humor," "bad temper," and "temperamental" all derive from this conception of human psychology.) Thus Cassius is saying that his bad humor had a physical cause.

Illustration by Leonhart Thurn-heisser, 1574, representing the four humours.