  We present the discovery and characterisation of an eclipsing binary
identified by the Next Generation Transit Survey in the $\sim$115 Myr old
Blanco 1 open cluster. NGTS J0002-29 comprises three M dwarfs: a short-period
binary and a companion in a wider orbit. This system is the first
well-characterised, low-mass eclipsing binary in Blanco 1. With a low mass
ratio, a tertiary companion and binary components that straddle the fully
convective boundary, it is an important benchmark system, and one of only two
well-characterised, low-mass eclipsing binaries at this age. We simultaneously
model light curves from NGTS, TESS, SPECULOOS and SAAO, radial velocities from
VLT/UVES and Keck/HIRES, and the system's spectral energy distribution. We find
that the binary components travel on circular orbits around their common centre
of mass in $P_{\rm orb} = 1.09800524 \pm 0.00000038$ days, and have masses
$M_{\rm pri}=0.3978\pm 0.0033$ M$_{\odot}$ and $M_{\rm sec}=0.2245\pm 0.0018$
M$_{\odot}$, radii $R_{\rm pri}=0.4037\pm 0.0048$ R$_{\odot}$ and $R_{\rm
sec}=0.2759\pm 0.0055$ R$_{\odot}$, and effective temperatures $T_{\rm
pri}=3372\,^{+44}_{-37}$ K and $T_{\rm sec}=3231\,^{+38}_{-31}$ K. We compare
these properties to the predictions of seven stellar evolution models, which
typically imply an inflated primary. The system joins a list of 19
well-characterised, low-mass, sub-Gyr, stellar-mass eclipsing binaries, which
constitute some of the strongest observational tests of stellar evolution
theory at low masses and young ages.
