I don't know if this will help, but I've found that often it's a technical issue. Some students will try to reach up to the black keys with there fingers. I find it very helpful to teach them that when a black key is played, we must move our elbows slightly forward and raise our wrists, otherwise the black keys feel like they're running away from us or always out of reach.
As for the reading part, there are some wonderful introductory pieces for sharps and flats in Lynn Freeman Olson's "Right From the Start." They can be quickly taught and if you keep them on the students' assignment sheets for a few weeks, they'll get it all figured out and never have trouble again!
_________________________
"If we continually try to force a child to do what he is afraid to do, he will become more timid, and will use his brains and energy, not to explore the unknown, but to find ways to avoid the pressures we put on him." (John Holt)
www.pianoped.com