Some aircraft dedicated for night-time operations were indeed painted black overall. However, consider when a black livery is actually useful. It only becomes really effective if your enemy is trying to spot you with his naked eye against a fairly uniform and dark night sky. If it’s a starry night, the silhouette of your plane moving against the stars, covering and uncovering them, is enough to give away your presence to an alert observer. Don’t get me wrong, even on a moon-lit starry night a black livery is useful, but it’s not game-changingly useful.

Black (or very dark) liveries come with a significant drawback, though. They are extremely visible against a blue sky. By applying a black livery you’re committing the aircraft to one (or two) of two roles: strictly night-time operations or training (that’s why some air forces today paint their training aircraft dark: for any supervising officers to be able to quickly spot the trainee’s plane in the sky).

And while not all bombers operated at night (not nearly so), let’s assume you’re willing to commit a number of bombers to working exclusively at night. Now consider the threat to a night-time bombing raid. (And here I’m making the assumption that we’re talking about the later phases of the war, and night-time bombings of Germany.)

First and foremost, a large formation will make the screens of enemy radars light up like a Christmas tree. Moreover, a formation of four-engine bombers makes an insane amount of noise. There’s no way a couple dozen, let alone more than a hundred, bombers flying wave after wave can sneak up on a city. The defenders will be ready. The bombers being black won’t help you one bit.

Then once a searchlight illuminates you, the black livery will again do nothing at all for you. The shape of the airframe will cause the light to “catch” on the curves and edges and you’ll be a sitting duck regardless of whether the plane is black or white, unless you managed to escape from the beam. Furthermore, once carnage over the flak field starts, explosions and fires erupting on damaged bombers will illuminate still more of them. The Germans actually developed a moderately successful tactic they called “Wild Pig” (Wilde Sau), which aimed to utilize this very phenomenon in order to allow “day-time” radar-less fighters to attack the bombers.

Still, with its focus on night bombing, the RAF did paint the underbelly (Edit. I probably should have said: lower surfaces, since the underside of the wings was also black in those planes.) of many of its heavy bombers matt black just in case, because obviously once you got away from the searchlight or before it even caught you, a white livery would once again become a massive vulnerability. It made no sense, however, to paint the bomber black overall, so the upper surfaces were painted in a more traditional camouflage pattern (Edit 2: These camouflage colours were designated ‘dark earth’ and ‘dark green’). The end result can be seen in this photo I took of one of two airworthy Avro Lancasters.

And day-time bombers, were, of course, another matter, entirely.

Not all bombers flew at night; most notably the USAAF operated almost entirely by day, but RAF Bomber Command (after December 1939) and the Luftwaffe over Britain (from late 1940) did indeed mostly operate by night, and both air forces painted the undersides of their aircraft black. Paradoxically when the Royal Flying Corps investigated night camouflage in 1918 it found that pure matt black all over was actually quite visible at night as it presented an obvious silhouette. They developed Nivo (NIght Varnish Orfordness), a dark grey-green that reflected light in a similar way to land or water, but by the late 1930s it was found to be too reflective in searchlights and phased out for matt black undersides with the standard temperate land scheme (dark green and brown) on upper surfaces as seen on e.g. the Short Stirling and Avro Lancaster. The Luftwaffe similarly, when switching from primarily day to night bombing in September 1940, painted the underside of its bombers black. Later in the war it developed a swirling grey wave pattern (Wallenmuster) used on the upper surfaces or sometimes all over bombers and night fighters, as can be seen on the Heinkel 219 at the Steven F. Udvar-Hazy Center.