The bacteria do not use the agar for growth, in fact the agar merely provides a semi-solid surface to grow the bacteria on. As for temperature, if you are growing bacteria on blood agar plates it usually implies that you are trying to culture bacteria which live in an animal. Therefore you would want to incubate the plates at the same temperature as the internal environment of the animal. Since our body temperature is usually 37 Celsius you would want to incubate the blood agar plate at 37 Celsius as the bacteria are accustomed to this temperature. When bacteria grow on an agar plate you will not see the individual bacterial cells.
Bacteria grow tremendously fast when supplied with an abundance of nutrients.
What you will see on an agar plate is a colony of bacterial cells. Within the colony are millions of individual bacterial cells. Colony morphology is a way scientists can identify bacteria. In fact there is a great book, commonly termed Bergey's Manual , which describes the majority of bacterial species identified by scientists, so far. This manual provides descriptions for the colony morphologies of each bacterial species.