  Blue supergiants are the brightest stars in their host galaxies and yet their
evolutionary status has been a long-standing problem in stellar astrophysics.
In this pioneering work, we present a large sample of 59 early B-type
supergiants in the Large Magellanic Cloud with newly derived stellar parameters
and identify the signatures of stars born from binary mergers among them. We
simulate novel 1D merger models of binaries consisting of supergiants with
hydrogen-free cores (primaries) and main-sequence companions (secondaries) and
consider the effects of interaction of the secondary with the core of the
primary. We follow the evolution of the new-born $16-40$ M$_{\odot}$ stars
until core-carbon depletion, close to their final pre-explosion structure.
Unlike stars which are born alone, stars born from such stellar mergers are
blue throughout their core helium-burning phase and reproduce the surface
gravities and Hertzsprung-Russel diagram positions of most of our sample. This
indicates that the observed blue supergiants are structurally similar to
merger-born stars. Moreover, the large nitrogen-to-carbon and oxygen ratios,
and helium enhancements exhibited by at least half our data sample are uniquely
consistent with our model predictions, leading us to conclude that a large
fraction of blue supergiants are indeed products of binary mergers.
