My question is how a species, given sufficiently advanced technology - could block out all - or most - visible light to a generally earthlike planet without rendering it uninhabitable.
The only caveat is that this should be a mechanism that could be 'turned off' if the generator were disabled.
edit - as far as not wiping out all life, I'm thinking the plant life is altered to survive on UV, and the atmosphere is engineered to allow more UV through.
You could try with a very large grid in space located at Lagrange point L1, creating a magnetically stabilized plasma mirror in vacuum.
The energy requirements would probably be staggering (you'd need an additional "ring" of solar stations or handwavium reactors), as the mirror would neet to be thousands of kilometers in size (at that distance, not much smaller in diameter than the planet itself); and it would be at risk of being disrupted by solar storms and the like. But for what I was able to find, it would be possible for such a mirror to have a plasma frequency just before the UV region, and the cutoff is pretty sharp.
It would not be possible to let IR radiation through though; a plasma mirror reflects everything below the critical frequency.
Of course, as soon as the plasma generator or the magnetic stabilization are turned off, or disrupted, the mirror would dissolve almost instantly.
A possible use case would be if the star was really luminous, to the point of rendering the planet uninhabitable if it all was let through. By increasing the cutoff frequency, more and more radiation is reflected away and the planetary surface cools down. This also has the effect of cutting off the lower frequencies, so the planet "daylight" is a dark violet-indigo.
A smaller mirror would make the Sun grow ever more luminous at sunrise and sunset, but it being lower in the sky would reduce the risks. You could perhaps have a vegetation that mostly photosynthesizes during two short periods of time, or preferentially accumulates intermediate compounds to maximize efficiency of the light phase to "digest" them during the night and day. Essentially there would be two short "days" (sunrise and sunset) and two longer "nights" (true night and indigo night).
A thick clouds generator could do the trick of blocking most of the light. However...
If we take as reference planet Earth (the only one we know where life exists), we see that most of the life is based on visible light.
Visible light is used by the plants to produce oxygen, and then the plants are the base of the food chain.
Smaller food chains are not based on light, but are pretty much limited to precise environments.
Therefore your requirement of
block out all - or most - visible light to a generally earthlike planet without rendering it uninhabitable
is basically unfeasible, as whatever mechanism would obscure the light would straightforwardly make the planet uninhabitable, with the exception of those environments where light-less food chains are established.
In theory you could produce and aerosolize enough very fine dust into the upper atmosphere to block out most, or even all, of the light that would normally penetrate to the surface of the planet. Depending on the particle size and the chemical make up of the material you used you could leave absorption gaps that allowed certain wavelengths of light through, these could be visible frequencies or in the non-visible parts of the Electromagnetic Spectrum. Getting the exact gaps you wanted would be more difficult than a pure black out.
Left to itself the material would eventually settle down far enough in the atmosphere to be caught up in cloud formation and rain out. So you couldn't flick a switch and have the effect end instantly but without some mechanism renewing the aerosol it would eventually (within a year or maybe two based on volcanic sulfur aerosol dissipation rates, it should be faster as solid dust particles rather than room temperature gas or liquid) disappear so you could still have a "saving the world" mission shutting down the renewal system.
The simplest solution is to put a star-shade in the Star-Planet L1 point, thus blocking the starlight reaching the planet.
This is actually a proposal to terraform Venus, by putting a small sunshade that would block enough of the Sun to make Venus cool down. You would still need to fix the atmosphere and probably soil, but putting a giant, ultra-thin film between a planet and its star is surprisingly feasible for an advanced space-faring civilisation.
The problem is, even if you use a shade that blocks only visible light, there is so much energy blocked that the planet will turn into an iceball, even assuming the plants have somehow been switched to infrared or ultraviolet photosynthesis.
To fix this, you have to make your shade from solar arrays, and use the produced energy to feed spotlight arrays on the other side, pointed at the planet. Those will emit infrared and/or ultraviolet in enough quantity to keep the planet hot.
Using solar arrays to feed lights, there will be losses. So the shade will have to be quite a lot bigger than what would be required to only cast a shadow on the planet. Make it two or three times as wide, to account for both losses and downtime of elements for maintenance.
Maintenance will be a big job. Solar arrays, power circuits and lights fail, micrometeorites punch holes in it and break things, radiation and solar wind erode components, and the thing is big enough to be unstable even at L1, so it will need many thruster arrays for station-keeping. But that should be feasible for your advanced civ as long as they are around, or their advanced automated maintenance systems are working.
Depending on the type of advancement, an array of panels could be placed all around the planet, to be removed at convenience.
Otherwise there is only smoke. Create a thick cloud screen of black soot and perpetual night pronto!
Of course, perpetual night is a vegetation-killer, there's no way around that. Plants die, plancton dies, chain food collapses, as temperatures in the world drop drastically. End of life.
