In the lower right hand corner of the screen, there is a symbol followed by a number. This number represents your Blood Count, often abbreviated BC. You can increase your Blood Count by killing mobs, but you may not receive a point for every mob you kill.
When your blood count reached 999, you will receive a message stating that a Blood Essence has been completed. The symbol will become a button, and when you click on it you will receive a Blood Essence.
Contrary to popular belief, you do not gain a Good Karma point whenever your Blood Count reaches a multiple of 100.