This worked, but we're wondering how to prevent this material from developing. The black coating on the leaves of your grapefruit tree is called sooty mold. This fungus doesn't hurt the tree, per se., but it is ugly. The honeydew coats the leaf and since the substance is high in sugars, the sooty mold fungus grows on it. The garden center person was correct, one way to get rid of sooty mold is to wash it from the leaves.
Copper kills the fungus, but it takes awhile for the black to weather off the tree.
The key to controlling sooty mold is to control the insects causing it. Aphids are common when the tree puts out its new growth in the spring and whiteflies are common during the summer. Of the insects I mentioned, I suspect whiteflies are causing the sooty mold. You can use an insecticidal soap, permethrin or pyrethrin, or a similar insecticide to control the whiteflies. Unless directed otherwise on the product label, treat the tree three times at 3 to 7 day intervals.