Steps
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Get Washington State data, http://www2.census.gov/geo/tiger/TIGER2010BLKPOPHU/tabblock2010_53_pophu.zip from https://www.census.gov/geo/maps-data/data/tiger-data.html under 2010 Census > Population & Housing Unit Counts - Blocks.
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Unzip file and used http://mapshaper.org/ to convert shapefile to topojson.
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Get geographic info about each block here: https://www.census.gov/cgi-bin/geo/shapefiles/index.php?year=2017&layergroup=Blocks+%282010%29
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Convert that to topojson too.