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Since the boot source is the hard drive, and it's turning black several seconds after startup, it sounds to me like the problem is with the hard drive. Have you tried starting up and pressing control are for recovery rate after starting the computer?
Of course, it could always be an LCD issue but an LCD issue would still allow the computer to boot. So if it were the LCD you would still see the computer appear to behave normally other than what you're saying – you would hear the hard drive running and you would hear it power up and stay powered up. Based on what if information you've given, I would say at this point that it's probably the hard drive. But if you are able to get into recovery it will check that for you and hopefully be able to tell you if it is the hard drive or something else.