For unknown reason, Apple does not allow an iMac Monitor to go completely black via the brightness control. Furthermore, the energy saver setting does not allow an interval less than 1 min. These can be annoying that the monitor keeps switching on while some others are VNC+ing the machine. Although there is no official "solution" from Apple, there is some free utility programs which can do the job. For example, this can turn the iMac screen completely black and can be recovered with a hotkey combination.