In 1948, when Aurora Avenue N (then U.S. Route 99) was being readied to connect to the under-construction Alaskan Way Viaduct, it was extended a block south of Denny Way to 6th Avenue and Battery Street, creating a short stretch of Aurora Avenue with no directional designation. This remained the case until 2019, when the replacement tunnel for the viaduct opened. At that time, Aurora south of Harrison Street reverted to its earlier name of 7th Avenue N, and since 7th Avenue south of Denny Way already existed, a new name was needed for the block-long Aurora Avenue.
According to The Urbanist, “Borealis,” referring to the aurora borealis, was a community favorite — but it was named after a nearby apartment building, not after the northern lights directly. As for why the names were changed in the first place, The Seattle Times reported that it was felt there were “negative connotations associated with [the name] Aurora Avenue” they wanted to avoid while reintegrating this stretch of the road into the neighborhood.