Why is there such a fine line between blue and green, and why is there the expression "The Blue Miracle"? Where does that come from?

Interesting question, due to a lack of my own specific knowledge, I have compiled the following.

Another interpretation comes from a user. According to him, the phrase originates from cloth dyeing. After dyeing, the green or yellow color reacted chemically with oxygen when the fabric was hung up, turning blue, so that the observer experienced " a blue miracle ," even though the cloth had previously been dyed differently.

https://www.redensarten-index.de/suche.php?suchkonzept=ein%2Bblaues%2BWunder%2Berleben&gawoe=an&sp0=rart_ou&sp1=rart_varien_ou

Blue…is the color stimulus perceived when light with a spectral distribution dominated by wavelengths in the interval between 460 and 490 nm enters the eye . Light with this property can be remitted as body color .

Due to its widespread use, blue comes in numerous shades, although the distinction between violet on the one hand and cyan or green on the other is rather blurred in German. In other language and culture areas, the same color spectrum is sometimes referred to differently. For example, in Russian, синий sínij and голубой goluboy are perceived as separate colors, and Italian even distinguishes three colors: blu , azzurro , and celeste , whereas German only knows one. Many shades of blue do have their own names, although these are usually not universally defined.

• Blue and green do not have their own color words in all languages , ^{[9] [10]} although the borderline case, turquoise , is also dependent on individual perception in German-speaking countries.

https://de.m.wikipedia.org/wiki/Blau

Green is the color stimulus that is perceived when light with a spectral distribution that almost exclusively contains wavelengths between 520 and 565 nm enters the eye. Light with this property can also be remitted as body color.

https://de.m.wikipedia.org/wiki/Gr%C3%BCn

The Blue Wonder , officially known as the Loschwitz Bridge , is one of Dresden's Elbe bridges . Built as a steel bridge between 1891 and 1893, it connects the two villages, or present-day districts, of Blasewitz on the left bank and Loschwitz on the right bank, which were incorporated into Dresden in 1921. In today's urban area, it is the easternmost fixed crossing of the Elbe .

https://de.m.wikipedia.org/wiki/Blaues_Wunder_(Dresden)