Blue-green is the region of the spectrum that most clearly demonstrates that colour categories are made rather than found.
A substantial number of languages, historically and today, use a single basic term covering what English separates into blue and green. Linguists call the category grue, an artificial word coined for the purpose. Ancient Chinese qing covers the range, and translators of classical texts have to decide case by case whether a qing dragon is blue or green. Japanese ao behaves similarly and still does in fixed expressions, which is why a Japanese traffic light that is green may be described as blue. Welsh glas historically covered the same ground. None of these is a failure to distinguish the hues; speakers discriminate them perfectly well and simply do not treat the boundary as a category boundary.
The materials reinforced the grouping. Turquoise, Egyptian faience and copper-based glazes produce colours that sit exactly on the line, and a culture whose prestige blue-green objects were all copper compounds had a good practical reason to treat the range as one substance-family.
Berlin and Kay's 1969 study placed the split between blue and green relatively late in the sequence in which languages acquire basic colour terms, after black, white, red and yellow. The finding has been refined and contested since, but the basic observation has held: where a language draws the line between blue and green is a fact about that language, not about the spectrum.