My dv8000 has a blank screen to but here's the good bit. When powering up it mementerily displays the "HP Invent" logo perfectly normally then goes blank. Allow the loptop to go to standby and when I move the mouse it comes back to life with a good display which then goes black after 5 seconds or so. No problem with external monitor.
You need to enter Bios (del, F2, esc, whichever key specified for your laptop), look for your external monitor or Display section, make sure it is set to LCD screen not external or Auto not External Only. This seems to be the problem.
Some laptops have a FuNction key for switching monitors, try that as a quick fix.