Black beetle? If you mean the lilies have yukky black deposits on them, they're the offspring of the Lily Beetle, which is actually bright red. You need to keep a close watch on the plants earlier in the year to catch and kill the beetles before they get a chance to lay any eggs.

Or perhaps pollen beetles? They are small and black and impossible to get rid of.

The pollen beetle is unsightly but, as far as I know, does not do any damage to plants.