Hey Hey!

it would absorb the heat but its got no where to disperses.. if anything it will make the room hotter. Its better for it to bounce of the walls and get removed by your fan... plus ul be loosing out on any reflective light on ur plants using black as a bass colour..

if your box in resonating by the vibrations from ur fan.. either put something heavy on top of the box and/or place nails or rubber feet on the bottom of the box.. this will disperse the vibrations.. also you could hang ur fan from some thick rubber bands. this will also absorb any vibrations considerably..