Intel-based Macs such as the iMac, Mac mini, MacBook, MacBook Pro, and Mac Pro rely on a combination of tones and blinking LEDs to display Power On Self Test (POST) error codes. Note that some newer Macintosh models only use tones to indicate error conditions. See this article for more information.
If the computer detects no RAM (Random Access Memory) or the RAM installed does not meet the appropriate specifications, the screen will remain black but the power LED on the front of the computer will blink once per second to signal the error. This error condition may be due to physically damaged RAM, installing the incorrect type of RAM, or not having RAM installed.
Some RAM may appear to pass the Power-On-Self-Test (POST) but still cannot be used by the operating system. In this case, the computer will display a gray screen, sound three tones and blink the power LED on the front of the computer three times, pause, and repeat the blinking until the computer is turned off.
The solution to both of these situations is to first re-seat the memory and test the computer again. If the memory fails the POST test again, try memory that has been verified to work correctly on another system (i.e., "known-good") or order new memory from the Apple Store.