"Literally, as the planet was consumed by near-total darkness, the clouds turning black"
This could be the effect of a frequency-shifting field. A spherical layer all around the planet has been turned into a nonlinear optical medium (in very limited volumes this can be done by magnetic fields, or it can happen in solid lattices, but in this case, well, magic - or sufficiently advanced technology), and it performs second-harmonic tuning on all photons comprised in the visible octave of 380 to 740 nanometers, shifting it into the 190 to 370 range, which means ultraviolet rays of A, B and C subtypes.
At the same time, plants have been reengineered genetically so that their clorophyll A and B molecules, already efficient enough in the blue and near ultraviolet range, are now packed differently and much more reactive in the full ultraviolet band. At the same time, their structure has changed so as not to be negatively affected by UV-A and UV-B, which is what would have happened before.
The viral vector that engineered the change was diffused over a period of several months, but its active phase was designed to trigger in the presence of significant quantities of UV-B rays. So, as soon as the frequency shifter field was activated, within a few hours all green plants "fell ill" and turned a sickly white, thereafter thriving on invisible light.
(The problem here is that staying in the open will now get you badly sunburnt).
A more convoluted transformation could perhaps convert visible light into microwaves, and have plants develop organic "rectennas" to capture their energy. Getting in the open will now just heat you up, just as if you were exposed to visible sunlight, except this would happen on a lesser level and on the whole body. Skin cancer would go down, but cataracts would go waaay up.