Answer: I think crown vetch might be your best bet--and it does have flowers (lavender and pink flowers, from June until frost). Peaceful Valley Farm Supply (ph# 916-272-4769, web site: http://www.groworganic.com) sells an seed mix called "Premium Erosion Mix", containing low-growing annual grasses, showy clovers, and 5% wildflowers. You could also try a regular wildflower mix--many will do well on relatively poor soils. Burpee carries a wildflower mix for the northeast, and a mix of perennial wildflowers.
As long as you have some soil over the shale, and the site isn't too dry, you should be able to get something established there. You will probably need to water the area in the first season, until the plants get established. Good luck!
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