a white surface is just as good. paint the walls white or buy some foam board at an art store or drug store. whatever you do, do not use aluminum foil, the crinkles cause light to reflect unevenly and can cause hot spots and burn your plants
Black absorbs light, you want flat white paint. But you have already painted black so it will take lots of coats of white unless you primer. maybe get on ebay and order some mylar.