The Black Cliffs provide year round climbing if you don't mind being cold in the winter. Summer days are really hot so spring and fall tend to be the best times a the Black Cliffs. The rock is all basalt. Access is easy and the cliffs usually aren't crowded. The guidebook Boise Climbs by Sandy Epeldi is a great thing to have if you are going to be climbing the Black Cliffs. You can pick it up at REI or Benchmark.
Take Highway 84 to the Gowen Rd/Highway 21 exit. Take Highway 21 towards Lucky Peak. The cliffs will be on both sides of the river. Park in the parking lots and choose a trail up.
Some of my best days at the black cliffs were on sunny cold winter days on the south facing cliffs and blistering hot days spent hiding in the shade on the north facing cliffs. The north and south sides have entireley different approaches. Consult Epeldi's book for the best published info for the south side approaches. The black cliffs can be a really fun place to climb, but it is by no means a destination. Out of the 100's of climbs, most are boringly similar. There are a handful of standouts though. The area is composed of numerous crags that are in close proximity. Digging in my memory for the standout sport climbs...Bologne Pony 10d-short cliffs, Unknown Leon 11a-short cliffs, Jet Screamin Hooter Queens 11b-tall cliffs, Pictures of Lily 11b-west car body canyon, Men Who Pause 11d-scary canyon, Beef Curtain 10d-scary canyon, Boise Gun Fighter 11d-scary canyon, The Groveller 11d-the dark side, Chassin a Snake 11d-the dark side, Sexual Underground 11c-easter island, Ehrlichman 12c-the nixon head, Watergate 13a-the nixon head.
A heads up to all Black Cliffs climbers: There is a lot of loose rock up there right now. I've had several holds break this spring and I've seen several sketchy loose blocks I don't remember being loose last season. It seems like the freeze-thaw cycles loosened some stuff this winter. Be careful out there!