"They are called Blue Zones because when Buettner and his colleagues were searching for these areas, they drew blue circles around them on a map." [1]

[1] https://www.healthline.com/nutrition/blue-zones

No, the author, Dan Buettner has a consulting practice around helping companies, orgs, municipalities implement the lessons from Blue Zones to help build healthier environments. https://www.bluezones.com/ Where the Blue came from I'm not sure.

My mind went to "blue haired grannies" https://tvtropes.org/pmwiki/pmwiki.php/Main/ElderlyBlueHaire...

This ^^ https://www.mentalfloss.com/article/643186/why-old-ladies-ha...

I believe it was because those areas were marked on a map using a blue marker.