For Blanchard Street, I can do no better than to quote Sophie Frye Bass, who in Pig-Tail Days in Old Seattle writes:
The name of Blanchard Street had long been a puzzle to me, but when I looked over some records of 1872 and found where John M. Blanchard had been one of the witnesses when my grandfather, Arthur Denny, platted a tract of land, then I knew.
Blanchard Street was part of A.A. Denny’s 6th Addition to the City of Seattle, filed in 1873, and indeed John M. Blanchard is one of the witnesses, along with Duncan T. Wheeler. Blanchard appears to have been city treasurer in 1879, as well as an insurance agent. Wheeler is referred to in 1871 as being a merchant, and in 1874 he and Blanchard apparently attended a masquerade ball dressed as members of the Ku Klux Klan(!), predating the establishment of an actual Klan chapter by half a century. (Their wives went as a “Canadian Squaw” and the “Queen of Chess,” respectively.)
Today, Blanchard Street begins at Elliott Avenue and goes just over half a mile to the northwest, where it ends at Westlake Avenue.