There is the checkbox "prevent black appearance". Then there is the solution of having a second object with a light material set not to emit light and set to no directionality to act as the visible part that will show up in reflections, to go along with the light object. Other than that, it is physically correct, since due to directionality no light rays from the light source actually hit the mirror, so do not get reflected. The workaround is relatively close to physical reality (where the physical reality is that mirrors, glass with imperfections, etc. actually bounce and bend rays outside of any perfect directionality, and those do reach the mirror - this could be set up in Corona I would say, but would be harder to set up and slower to render than the "have two objects, the light to emit light with directionality, and another to act as the visible part of the light")
