Here are a couple of suggestions. Spray paint flower pots black then pop in a 4 inch plant with white flowers from the plant nursery. You might be able to get the nursery to order them for you. White flowers that should be available at a reasonable price include: gerbera daisy, geraniums, miniature roses, and petunias.
Another idea would be clear vases half filled with black marbles and one spray of white orchids. Or use round clear vases with a single floating white flower. Place the vase on a black charger plate (those plates that are bigger than a dinner plate and used under the dinner plate) place white votive candles around the vase on the charger plate.
Use black lace that you've bought at a discount fabric store - seconds are fine - nobody will notice a glitch in the pattern, as a runner for each table over a white table cloth. Place a simple bouquet of white airy flowers like mini carns, mini roses, baby's breath and green leaves in a clear vase on the runner.
Flowers don't naturally occur in black, but you can use floral spray which does come in black to 'paint' baby's breath. Then mix the black baby's breath in with real white flowers.
The challenge with black is that it can go gothic or Halloween fast.