Was wondering this the other day. Assuming Germany or Japan had the capability to hit the mainland US with bombers during the 2nd World War, how effective would a city wide blackout have been? It seems to me that the pilots would have maps of the area, and that they could use their rate of travel divided by total distance and figure out, fairly accurately, where they were in relation to where they should be.
That being said, would a blackout have actually been effective in preventing the bombers from hitting their targets?
Sub-question: Given the technology of the time, weren't bombers already fairly inaccurate? Would it actually have mattered if they couldn't see exactly what they were bombing?
The potential effectiveness of a US blackout would depend on numerous factors, probably better for /r/historywhatif/
Regarding the actual effectiveness of blackouts, principally in the UK and Germany, early war bombers were indeed fairly inaccurate, even during daylight, but there's no sense in making things easier for the enemy; navigation by dead reckoning was far from straightforward, especially if manoeuvring to avoid flak or night fighters, and the lights of neutral Ireland were a useful navigational aid for the Luftwaffe. Improvements in navigational technology over the course of the war such as radio beam guidance (German Knickebein and X- and Y-Gerät, RAF Oboe and GEE) and ground scanning radar reduced the effect of the blackout on navigation, but were not infallible, being subject to electronic countermeasures. The blackout also increased the effectiveness of decoy sites, used widely in Britain and Germany, lights and fires set up in open country near actual targets; a decoy version of the Krupp works outside Essen attracted around three-quarters of the attacks aimed at the real plant, according to German air force estimates in 1941.
You would be better asking how effective Axis blackouts were at preventing Allied bombers from striking their targets.
The blackouts were less from stopping the bombing and more about making the bombing more difficult. At night, the illumination of cities would allow for easier navigation.
As you have guessed, the bombers were inaccurate to begin with. The B-17 was equipped with the Norden Bombsight which was reputed to allow the crew to drop a bomb into a barrel. In reality, it was useless. A large reason why so many bombers were assigned per sortie in WWII compared to Vietnam and after was due to the inaccuracy.
The idea of accuracy(via the Norden sight) was dropped in favor for area saturation where they assumed that a large proportion of bombs would miss the target. They sent out a large number of bombers so that the "required number" of bombs would hit the target(bridge/factor etc). Some targets would require multiple sorties.