Yellow traps won't work with black fly which are actually aphids, most of whom don't have wings. If you mean black flies, eg fungus gnats, no, they're not attracted to yellow, they're looking for humus-rich compost in which to lay their eggs - whereas whitefly look for young foliage and therefore are attracted to yellow. But the yellow flypapers don't discriminate and also trap lacewings and other useful predators.
I find the most effective thing is to use the small gentle upholstery brush on my vacuum cleaner to vacuum clean then off. Run it gently along the underside of the leaves, starting with the most infested. Best early in the morning when the air is cool and the whitefly don't have the energy to take off at your approach. You'll need to repeat, say two days running and thereafter as needed. It doesn't completely eradicate, but it does give about 95% control.
If you have a heavy infestation, some of the leaves will have a dense covering of transparent green scales on the underside - these are the baby whitefly. Best to remove these leaves before they hatch out.
Sprays aren't particularly effective. You probably need a systemic, and they're not safe to humans. I believe there is a biological control available, but that's only doing the same job as the vacuum cleaner, a good deal more slowly. Butterwort is an insectivorous plant with leaves in the right pale green colour to attract whitefly and may help.