If you have a Compaq CQ50 laptop that blackscreens on boot or login, try hitting fn+F4. This combination toggles the display mode from the monitor to the remote screen (whether you have one connected or not). I'm not sure how or why this malfunctions, but I struggled with this problem, rebooting, reseating HD and RAM, and anything else I could try for a couple years until I discovered this a couple days ago. So far it has corrected the problem every time the laptop blackscreens (which it has been doing more frequently for some reason).
Hope this works for you.