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A Geroch-Bekenstein engine is a theoretical/imaginary mechanism that uses a black hole to reduce entropy, i.e., to make unusable energy usable (i.e., akin to Maxwell's demon). The engine was described by Jacob Bekenstein in 1972 based on an idea by Robert Geroch. It is not practical, merely a thought experiment exploring possible black-hole phenomena, e.g., what happens when something with entropy falls into the hole, and its concept has provided motivation for further black hole theory and speculation, including black hole thermodynamics. The mechanism (the Geroch process) lowers a container of high-entropy EMR toward the event horizon, attached to a wire, capturing the mechanical energy due to the pull on the wire by gravitational on the container's and contents' mass, then allowing the EMR to escape into the black hole while the container is just above the event horizon. The container is lifted again using the captured mechanical energy, and the additional mechanical energy due to the weight of the released EMR remains usable.
The concept also constitutes a plausibility argument that mechanisms recovering difficult-to-use energy that might exist around actual black holes, but I doubt it is seriously considered a means to reduce total entropy, even in theory. It has been speculated that a magnetically arrested disk (MAD) might resemble the engine sufficiently to make use of energy that would seemingly be unusable.
Among the inspirations for the Geroch engine was the concept of Wheeler's demon, ascribed to John Wheeler, that to our point of view, something with entropy tossed into a black hole gets rid of the entropy.