In this setting materials science is decades ahead, so answers should be limited by technology that isn't too speculative and don't presume anything supernatural beyond what I describe in this question.
Resources can be assumed to be very abundant. A lack of cultural value placed on precious metals combined with greater abundance and better exploitation also makes precious metals a way cheaper. This is way wealthier than any modern society.
People in this world have the means to create magical darkness through magical enchantments: This darkness absorbs all radiation except non-relativistic atoms without producing any corresponding heat, but it does not otherwise interact with matter physically and only consumes heat via absorbing photons. Some of the darkness is used up when absorbing radiation, so absorbing too much mass or energy worth of radiation will require a prohibitively expensive enchantment (though this shouldn't be an issue except when dealing with supercritical events, or trying to absorb a significant mass of plasma).
The other relevant things the magical darkness can be used for is short range teleportation (entirely within the magical darkness), and physically separating non-solid atomic matter by desired qualities. It can easily physically separate the components of a gas or liquid. It can also replenish nuclear fuel rods while in use with teleportation, or sort objects according to simple rules. Using magical darkness for teleportation is costly however and so you wouldn't want a radiator/heatsink design that regularly had components teleporting around.
Importantly the magical darkness is physically intangible, but it can only co-occupy with gas or any material transparent to visible light.
So it would seem like using water would be the be an obvious choice due to its heat capacity, thermal conductivity, and transparency. However, with nearly any desired isotope being affordable surely you could do better by using heavier isotopes for the water right? Plus there's plenty of materials like molten glass that would work as they are technically transparent even if you can't tell because of their glow. There's also the consideration of what kind of radiator setup to flow the clear liquid through (maybe a highly thermally conductive aerogel?) to maximize cooling, since normal radiators don't have the liberty of just deleting heat.
Though I might be totally wrong about the most efficient general design and maybe it's better to use something else to carry the magical darkness like:
Supercritical CO2: It could allow for way more surface area to be in contact with the magical darkness, plus you could make really tiny channels for it to flow through within the device being cooled maybe?
Solid diamond: Very thermally conductive and clear, could also be pair with a transparent liquid.
Water vapor: Might have more surface area to emit IR?, or does that not apply if the whole bulk of material can shed IR? to carry the magical darkness.
How effectively could a heatsink designed around radiating IR into magical darkness be compared to a normal heatsink in terms of quickly pulling heat from a given volume/mass, and what would it be designed like broadly speaking?
Your answers should particularly focus on compact heat sinks that do not mess up the aerodynamics of what they're attached to like normal radiators would.
I strongly suspect that getting a massive surface area heatsink to shed IR into darkness would let you get orders of magnitude better performance in terms of heat moved per second per volume/mass than just using normal radiators shedding heat into the adjacent environment. After all these shadow-radiators wouldn't be limited by transfer of heat to the surroundings, but by surface area, which seems like something that could be exploited to a way greater degree given we can create things like microchips that are absurdly finely detailed.
I included the hard science tag because I realized this heatsink will play a crucial role in my setting and I need citations and/or math so I can actually know how well I could use it various applications. Extremely compact and effective heatsinks are of great importance for my setting as people want to use small nuclear reactors or Project Pluto style engines for as many things as possible and with good radiation shielding, heat management becomes the biggest obstacle to that (their technology and magic make fallout a non issue, and the environment is already barren of nearly any life such that fallout will increase the biomass). Plus conflict occurring in an Antarctic environment means that thermal imaging is particular effective and it would be profoundly useful to be able to actually hide from IR.