Stars with masses greater than about ten solar masses are thought to end their lives either in a supernova or in a direct gravitational collapse process, either of which could have a black hole as a remnant. But there is as yet no direct observational evidence to support either gravitational collapse in general or the formation of black hole remnants in particular. Here we report a large overabundance of oxygen, magnesium, silicon and sulphur in the atmosphere of the star orbiting a probable black hole in the binary system GRO J1655-40 (also known as Nova Scorpii 1994). These α-elements are six to ten times more abundant in the star's atmosphere than they are in the Sun's. We interpret these high abundances as evidence for supernova ejecta captured by the companion star. The relative abundances of these elements suggest that the supernova progenitor was in the mass range 25-40 solar masses.