Not much is known about blue whale songs although most researchers think that they help males woo their mates, as is the case with closely-related species. That can make any modifications to a whale melody fairly important biologically. If two populations cannot talk to each other over time, they're going to grow apart.
Eventually, populations with different takes on a tune might splinter into subspecies, with their own behaviors and quirks. There's not yet evidence to show that has happened with these blue whales, nor much information on what might have driven them from their southerly cousins. But even if the whales in the new group don't yet formally occupy a new branch on the tree of life, they are worth getting to know.
What this discovery shows us is that there are different populations, with different adaptations, with potentially different needs. Moreover, the potentially restricted range, intensive historic whaling, and the fact that the song-type has been previously undetected, suggests a small population that is in critical need of status assessment and conservation action.
Why Are Blue Whales So Gigantic?
Short answer: Whale gigantism is tied closely to two things, 1. their choice of prey, and 2. the coincidence of their evolution with a global increase in the upwelling of nutrient-rich water from the depths of the ocean.