Where does “grey-blue” come from?

grey-blue (English) comes from English Blue, from Middle English blēwe, from Anglo-Norman blew, from Middle French bleu, from Old French blöe, from Latvian blāvus, from Old High German blāo, from Proto-West Germanic blāu — yellow, blond, grey.

Ancestry of “grey-blue”, step by step

grey-blue traces back through two separate lines of ancestry.

via English Blue

StepLanguageWordMeaning
1EnglishBlueA letterman at Oxford or Cambridge; A member of...
2Middle Englishblēwe
3Anglo-Normanblewblue
4Middle Frenchbleublue
5Old Frenchblöeblue
6Latvianblāvusaccusative plural masculine form of blāvs
7Old High Germanblāoblue, dark, grey; blue; yellow
8Proto-West Germanicblāublue
9Proto-Germanicblēwazblue; a dark bluish or grey colour, black
10Proto-Indo-Europeanbʰlēw-yellow, blond, grey

via English Grey

StepLanguageWordMeaning
1EnglishGreyA member of the Royal Scots Greys, a cavalry...
2Middle Englishgreygrey, dull, drab; glinting, glistening; grey
3Old Englishgrǣġ
4Proto-West Germanic*grāugrey
5Proto-Germanicgrēwazgrey
6Proto-Indo-Europeanǵʰreh₁-to become green, grow; to grow; to green, to grow
Every word from Proto-Indo-European bʰlēw-Every word from Proto-Germanic blēwazEvery word from Proto-West Germanic blāu