I cannot speak for the schools in Blackhawk and Alamo, but they are nice areas albeit very different. Blackhawk is much newer construction.
The gentleman below mentioned that there is a portion of Walnut Creek that feeds into Lafayette schools. You may want to look at the houses that are along Olympic Blvd., West of Hwy 680. That's the general area that feeds into the Lafayette schools (although you'd want to confirm any house w/the owner/realtor).
I'd say that another thing to take into consideration when you look at the three towns is location. W/Lafayette being on a BART line, it's far easier to get into SF during a work day than Alamo & Blackhawk. If you're at home but thinking about going back to work some time in the future, that's something to think about. You can get to SF, Oakland, Walnut Creek, San Ramon/Bishop Ranch and Pleasanton all w/in 45 minutes either driving or taking BART.