To be a “true blue” something is to be a waschechter something in German.
DWDS had a few examples of this in use:
ein waschechter Macho – “a true Macho” – yes, you can be A Macho in German (dict.cc offers “himbo” as a translation and this does capture the pejorative sense of the word).
eine waschechte Kulturstadt – “a true culture city” – perhaps best understood as being derived from the European City of Culture designation.
waschechten Mozartfans – “true Mozart fans” – it would seem that almost anything can be combined with Fan, as in English.
ein waschechter Verlagskontrakt – “a genuine publishing contract.”
When we take waschechter apart we get wasch – “wash” and echter – “more real,” “truer,” “more typical.” The waschechter combination can also mean “colorfast,” from which likely comes the implication that afforded the figurative use: that the characteristic is one that won’t “fade” or “disappear” (our own phrase “true blue” has this origin, coming from a time period where most blue dyes were fugitive). Intriguingly, in researching this I came across two phrases using gewaschen – Er ist mit allen Wassern gewaschen meaning “He’s a smooth customer” – and mit allen Wassern gewaschen sein – “to know every trick in the book” or “shrewd.” Which leaves me wondering what it might mean if you had washed everything in, say, Sekt or Chanel Number 5.