Black wrap (a sort of light-absorbing aluminium tape used in the entertainment industry) if you can scrounge a bit of it. Or a black cloth-effect bar tape. Possibly you might have better results trying to shield your eyes from the reflected light than reducing the amount of light reflected - I'm imagining some sort of hood for the light.
But getting the cables out of the beam would be my preferred approach, for obvious reasons. I have this problem on my mountain bike, with the front brake hose (standard not particularly shiny black colour) getting in front of the only suitable bit of handlebar for mounting a light. I ride that so little in darkness that I consider it a mild irritation. My other bikes have fork crown and boom mounted lights, which avoid the problem (though foot flash[1] is a spiritual relative). Perhaps use a SpaceGrip or similar to move the light further forward?
[1] On which note, mounting the light further forward of the bottom bracket shell is a better way to reduce foot flash than eg. wearing dark-coloured shoes. The rookie mistake is to mount the light somewhere directly above the bottom bracket in the interests of height.