Why is blood agar considered to be differential media?

Blood agar is considered to be differential media. This is because of its ability to display three distinct forms of hemolysis onto a plate. These forms consist of alpha-hemolysis, gamma-hemolysis, and beta-hemolysis. 

• Alpha-hemolysis gives off a green discoloration surrounding bacterial colonies. 
• Beta-hemolysis is complete hemolysis which results in a transparent area surrounding the colonies. 
• Gamma-hemolysis results in no hemolysis, and there is no difference in the appearance of the agar surrounding the colonies. 

Blood agar is not classified as selective media since it does not have any unique inhibitory substances which selectively target specific types of bacteria, which are the attributes of selective media.