I had the exact same problem you did !
I have an ibook 700 mhz g3 too
''Screen just went blank completely (no back light no nothing, tried hooking it up to another monitor...nothing!!!). Now sometimes it will boot sometimes it won't. Managed to do a hardware test on it and everything came out fine (but I am sceptical). Now it makes 'booting' sounds but the screen is completely dead. I can make it sleep by pressing the power button and then 's' so I know something is going on back there but that is it. On a couple of occassions it has looked like it was starting up and then the screen starts flashing grey and run lines down the screen. What should I do? (yes of course it is out of warranty...sob(apple is good like that)).''
I did all you did , did you find a solution ? If so, please let me know !!
I tought my computer died coz I dropped it a couple of moths ago, is good to know Im not the only one with that issue.
I must say a friend of mine had exactly the same problem...so its not a coincidence, is this a known issue??
+ what is the logic board program ?