  Recent evolutionary computations predict that a few percent of massive OB
stars in binary systems should have a dormant BH companion. Despite several
reported X-ray quiet OB+BH systems over the last couple of years, finding them
with certainty remains challenging. These have great importance as they can be
gravitational wave (GW) source progenitors, and are landmark systems in
constraining supernova kick physics. This work aims to characterise the hidden
companions to the single-lined spectroscopic binaries (SB1s) in the B star
population of the young open Galactic cluster NGC 6231 to find candidate
systems for harbouring compact object companions. With the orbital solutions
for each SB1 previously constrained, we applied Fourier spectral disentangling
to multi-epoch optical VLT/FLAMES spectra of each target to extract a potential
signature of a faint companion, and to identify newly disentangled double-lined
spectroscopic binaries (SB2s). For targets where the disentangling does not
reveal any signature of a stellar companion, we performed atmospheric and
evolutionary modelling on the primary to obtain constraints on the unseen
companion. Seven newly classified SB2 systems with mass ratios down to near 0.1
were identified. From the remaining targets, for which no faint companion could
be extracted from the spectra, four are found to have companion masses in the
predicted mass ranges of neutron stars (NSes) and BHes. Two of these have
companion masses between 1 and 3.5 $M_{\odot}$, making them potential hosts of
NSes (or lower mass main sequence stars). The other two are between 2.5 to 8
$M_{\odot}$ and 1.6 and 26 $M_{\odot}$, respectively, and so are identified as
candidates for harbouring BH companions. However, unambiguous identification of
these systems as X-ray quiet compact object harbouring binaries requires follow
up observations.
