  In a failed supernova, partial ejection of the progenitor's outer envelope
can occur due to weakening of the core's gravity by neutrino emission in the
protoneutron star phase. We consider emission when this ejecta sweeps up the
circumstellar material, analogous to supernova remnants (SNRs). We focus on
failed explosions of blue supergiants, and find that the emission can be bright
in soft X-rays. Due to its soft emission, we find that sources in the Large
Magellanic Cloud (LMC) are more promising to detect than those in the Galactic
disk. These remnants are characteristic in smallness ($\lesssim 10$ pc) and
slowness (100s of ${\rm km\ s^{-1}}$) compared to typical SNRs. Although the
expected number of detectable sources is small (up to a few by eROSITA 4-year
all-sky survey), prospects are better for deeper surveys targeting the LMC.
Detection of these failed SNRs will realize observational studies of mass
ejection upon black hole formation.
