What does the Spanish term 'principal azul' mean?
If you mean "principe azul" then it is the English equivalent to "knight in shining armor". The whole "dream guy" ideal. If not, then I don't think it has any meaning other than the one you have.
I suspect that you mean "príncipe azul". This term gets tossed around on Spanish language TV "reality" shows (similar to "The most Eligible Bachelor" [or whatever it is in English]) and carries the same overtones as "shining knight" or "dream prince".
principal azul does not make sense to me. Can you give us more context?
Principal Blue seems to be a Place or a trademark.
Unless you mean el príncipe azul = the blue prince