The Short Version
This claim is true. The blue whale, a baleen whale, is widely recognized by authoritative sources—including Britannica, NOAA Fisheries, and Guinness World Records—as the largest animal ever to have lived on Earth, measured by mass and overall body size. The phrasing "some species of baleen whales, including the blue whale" is logically satisfied by the blue whale alone. The only minor caveat is that by linear length, the bootlace worm exceeds the blue whale, but "largest" conventionally refers to overall size, not length.