library(UScensus2010)
library(UScensus2010tract)
library(UScensus2010blkgrp)
library(UScensus2010blk)

Cook_tracts <- UScensus2010::county(state = "Illinois", name = "Cook", level = "tract")
Cook_tracts@data[1:8, 1:8]
plot(Cook_tracts)

Cook_blkgrps <- UScensus2010::county(state = "Illinois", name = "Cook", level = "blkgrp")
Cook_blkgrps@data[1:8, 1:8]

Cook_blks <- UScensus2010::county(state = "Illinois", name = "Cook", level = "blk")
Cook_blks@data[1:8, 1:8] # FIXME: yields "NA" just about everywhere, including within row.names