How do I fix my Macbook's dark screen?

I tried booting the computer off a bootable backup, and while everything else worked, the screen was still dark. Reset your PRAM, and then do that again. If, even with fresh PRAM and a different boot disk, your screen is still unlit, then it's for certain a hardware problem, at least. Usual causes for a black screen: a burnt-out backlight, fried power inverter (tiny circuit) or a crimped cable pinched in the hinge. If they looked at it already, and there there's no magic-wiggling angle at which the light does come on for a moment, you can eliminate the inverter and probably eliminate the crimping too. I have to think they tested for a burned out backlight, but who knows. And no, a "failed logic board" could fail in just one certain way, because when they say that, they almost always mean that one tiny component in one dark corner is dead and needs replacing. But Apple (like most modern manufacturers) doesn't even bother to isolate and fix anything by soldering individual components anymore. They don't even stock replacement components, just entire motherboards, so that's what they replace. It's more cost/inventory effective for them that way, even if they're often replacing a $400 motherboard because of a four cent resistor most of the time. Not great news, since you've tried already, but compared to your average shady electronics shop, the Apple Store is probably the least likely to rip you off: they have no real incentive to do so.

i used tekserve before the apple store came to new york and would recommend it. http://www.tekserve.com/

I'd try http://nycipoddoctor.com/ guy featured on this story http://www.npr.org/templates/story/story.php?storyId=106876617.

Oh no. Resetting the SMC didn't work. Resetting the PRAM and then booting off the bootable backup didn't work. There's no magic wiggling angle that works. I'm still hoping (probably foolishly) that someone'll have a solution that magically works, but it's not looking good.

Did you repair disk permissions after deleting the .mkext files?

I've had positive service experiences with both http://www.tekserve.com and http://www.digsoc.com

Sounds to me like a backlight bulb (I've had this happen twice.) If they checked the inverter, I can't imagine that (or a simple crimped wire) was the problem. I'm really surprised they didn't take an explicit look the backlight, though. Maybe ask the Apple store whether they've ruled it out, or perhaps take it to one of the better local stores suggested above? I mean, motherboard issues are certainly possible, but backlight is definitely something to rule out first. I've had mixed experiences with Apple Store and Apple repair folk (rather like computer repair folk in general): sometimes they are awesome, and sometimes I really obviously know much more than they do, and they miss really obvious things like checking your backlight. If your computer is out of warranty, it's do-able to swap out screen components yourself, buying replacements off of eBay (or finding them elsewhere.) It's finicky, and I don't suggest doing it in the middle of an ill-lit party, as I did, but if you've got a steady hand and some confidence in taking computers apart, it might end up being quicker as a diagnostic test than ferrying it back and forth to unresponsive Apple store people. There are very clear instructions for almost every model imaginable online.

I've heard good things about http://www.macsupportstore.com/ in Brooklyn.

No the bulb will most likely outlive you. It is the inverter board. And will cost you a couple of hundred to repair, including labor. FYI Check to be sure you do not have AppleCare. some systems, especially those purchased through schools came bundled. You can go to the Apple site and punch in your serial number.