Spoilers for 30 Nights!
One of my criticisms of 30 Nights is that while some cause of the blackouts is hinted at through the backstories of a couple of NPCs and the actions of some of the factions, the plot never has the PCs getting to the bottom of it, even in its final chapter where they turn on the lights. Like you, I came up with my own story that I think is canon based on what I could learn from 30 Nights, Cutting Black, and whatever else was out there. It's entirely corporate-caused, to punish the UCAS for ending corporate extraterritoriality. The "bombs" if you want to call them that originate in the matrix of the affected cities, affect the power grid, and everything connected to the matrix, knocking them out in a one-time "blast" that was like hitting a kill switch on every modern device. Something like a matrix-based jammer remains in the power grid host, now inaccessible except through direct connection, that prevents it from being turned back on.
That's my head canon and I think it's pretty close to official canon. I use that story to explain why some devices could still be found working - they were behind on their driver updates, they're glitchy and never worked right, they were off at the time of the blast, they were custom built without corporate parts, they came from out of town, that sort of thing. And about halfway through the campaign we say that more and more devices are becoming operational, because some people have found a way to circumvent the problem, or they're getting shipments from out of town, etc, cause it's ok to take away the tech for a little while, but the setting demands that they return so your deckers and riggers have things to do and challenges to face.