Blue is the colour people reach for when they want a phone to stop shouting at them, and they are right to. It is also the colour most likely to end up looking flat, dull or oddly cold, because blue behaves differently on a screen than it does anywhere else.
Here is what makes a blue wallpaper restful rather than lifeless, and which shades hold up.
Why blue reads as calm
Partly convention and partly physiology. Blue is the colour of distance in almost every landscape, so the eye reads it as far away and therefore as not requiring attention. Warm colours do the opposite, which is why every warning label in the world is red or amber.
For a lock screen that matters more than it sounds. You look at the screen to check the time and you want to stop looking. A colour the eye files as background lets you do that. A colour it files as foreground keeps pulling you back.
The shades that work, and the ones that do not
Deep navy and midnight. The most reliable. Dark enough for a white clock, saturated enough not to read as grey, and close enough to black to be cheap on an OLED screen.
Teal and blue-green. The most comfortable to look at for long periods, because it sits near the middle of the visible range where the eye works with the least effort. Underwater scenes land here naturally.
Pale sky blue. The difficult one. It looks calm in a gallery and turns into a bright, featureless field on a phone, with a white clock lost in it. Usable only with a dark clock colour.
Electric and cyan. Not restful at all, whatever the colour theory says. High-saturation cyan is as demanding as any warm colour, and it is the shade most likely to be replaced quickly.
The rule that covers all four: value matters more than hue. A dark blue of any shade is easier to live with than a light one, and the reason is the clock rather than the colour.
The gradient problem
Blue is where compression damage is most visible, and it is the single most common reason a blue wallpaper looks cheap.
A large area of smooth blue — open water, a clear sky, a fade from navy to black — contains almost no detail for a compressed file to hide behind. So the file saves it as a series of steps rather than a smooth transition, and you see bands.
They are invisible at full brightness in a bright room and obvious at the low brightness you actually use at night, which is why people notice weeks later. Start from a large original rather than something re-saved on the way to you.
Your screen is warming the blue without telling you
True Tone adjusts the display's white balance to match the light in the room, and Night Shift warms it further on a schedule. Both push blue towards grey, and both are usually on by default. So the wallpaper you chose in a gallery under a neutral profile is not the wallpaper you are looking at in the evening. If a blue image seems duller on your phone than it did when you picked it, check both settings before deciding the file is wrong. It is worth looking at any blue wallpaper once with them off, so you know what you actually have.
A quick test for whether a blue is restful
Colour theory is a poor guide to what you will actually tolerate, so here is a practical check that takes a minute.
Set the wallpaper, lock the phone, and then use it normally for an hour without thinking about it. At the end of the hour, ask whether you looked at the wallpaper or only at the clock.
A restful blue is one you stopped seeing. You picked up the phone many times, read the time, and never registered the picture. That sounds like a criticism and it is the entire goal.
A demanding blue is one you kept noticing. Not disliking, necessarily — noticing. Each time the screen came on, some part of your attention went to the image before it went to the time. That is the wallpaper you will replace, usually without being able to say why.
The shades that fail this test are almost always the bright and highly saturated ones, which is why pale sky blue and electric cyan appear on the difficult list above despite reading as calm colours in the abstract.
Where underwater scenes fit
Water is the most natural source of blue that is not a flat gradient, which is exactly what the banding problem needs.
An aquarium or ocean scene gives you blue with structure in it: light shafts, particles, movement, the fade from surface to depth. That structure both hides compression and gives the eye somewhere to rest without demanding anything, which is a combination flat colour cannot provide.
It also solves the clock question by default. Water gets darker with depth, so a scene shot looking downward has its darkest area at the top of the frame, which is where the time goes.
Living with one
Blue is the easiest colour to pair with a home screen, because app icons are mostly warm and the contrast works in your favour rather than against it.
Blur the lock screen image in the Home Screen panel under Customise and you get a blue field that icons sit on cleanly. This is one of the few cases where the blurred version is arguably better than a second image.
Quick answers
Why does my blue wallpaper look grey?
Most likely True Tone or Night Shift, both of which warm the display. Failing that, the file may have desaturated blacks from compression, which lifts the whole image towards grey.
Is a dark blue as good as black for battery?
Close, on an OLED screen. A blue pixel is lit where a black one is not, so navy costs slightly more than black and far less than white. The difference between navy and black is small enough to ignore.
Which blue is best for a Sleep Focus screen?
The darkest one you have, and ideally one without a bright element in it. You are looking at it in a dark room, and anything light is uncomfortable at that hour.
Does blue actually help you sleep?
Blue light in general is the thing usually discussed, and that is about the display's overall output rather than the colour of the picture. Night Shift and screen brightness do far more than the wallpaper's hue.
Where to get it
Aquarium 3D: Live Wallpapers is built almost entirely from blues, from shallow teal to near-black deep water, previewed in a phone-shaped frame so you can see how a shade sits under the clock before you save.
Pick a dark blue with structure in it. Flat pale blue is where this goes wrong.