It might not be. Assuming you could find a black hole which is not actively engaged in swallowing something (and hence spewing out huge amounts of heat and relativistic particle jets), you could surround it with mirrors to create an internal volume with a very low photon temperature at relatively (in astronomical terms) mild expense.
Obviously a spherical shell would be unstable, but you don't need a contiguous shell to lower the internal temperature. A rotating ring (or easier, an orbiting series of small, overlapping satellites) would be stable (not in the orbital sense, but in that they wouldn't need to hold themselves up) and could use the thermal gradient between their sides to power their station-keeping. You could construct additional rings at greater distances, each with a different inclination, to cover more of the angular area.
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It might not be. Assuming you could find a black hole which is not actively engaged in swallowing something (and hence spewing out huge amounts of heat and relativistic particle jets), you could surround it with mirrors to create an internal volume with a very low photon temperature at relatively (in astronomical terms) mild expense.
Obviously a spherical shell would be unstable, but you don't need a contiguous shell to lower the internal temperature. A rotating ring (or easier, an orbiting series of small, overlapping satellites) would be stable (not in the orbital sense, but in that they wouldn't need to hold themselves up) and could use the thermal gradient between their sides to power their station-keeping. You could construct additional rings at greater distances, each with a different inclination, to cover more of the angular area.