Could be something to do with Blue Angel being a German eco organisation that rates products. Blue is certainly a term that has been applied to heating boilers for some time.
At a guess a reference to the blue of an unpolluted sky rather than the green of growing plants.
Which Euro standard (Euro VI, Euro VII etc)is going to bring this in?
No idea but passenger cars with adblue diesel engines are definitely already in use in some countries so can't help but assume the EU will make that compulsory just like all these other stupid standards