NGTS clusters survey - III: A low-mass eclipsing binary in the Blanco 1 open cluster spanning the fully convective boundary

We present the discovery and characterisation of a P  ~ 1.1 day, low-mass eclipsing binary (EB) found by the Next Generation Transit Survey in the ~115 Myr old Blanco 1 open cluster. The system is the first well-characterised EB in Blanco 1; it has a low-mass tertiary companion, a low mass ratio, and components that straddle the fully convective boundary. 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 compare the system properties to the predictions of seven stellar evolution models. This new EB joins a list of 20 well-characterised, low-mass, sub-Gyr EBs which constitute some of the strongest observational tests of stellar evolution theory at low masses and young ages.