Code blue is not literally used everywhere... many places in the US use a number these days like 99 for "Blue" and then other memorable two digit numbers for more specific, less emergent needs like respiratory support or rapid response. Same thing.
There is both the PA codes, ie anything from fire (unsurprisingly often code red), active shooter, to rapid response (person still has a pulse but ain’t feeling so hot)... to full on code blue.. but most places I work these are “quick” numbers like 66 or 88.
I don’t think freaking out bystanders is a consideration.. since the non medical codes are often posted prominently in conspicuous locations, and the medical ones.. I mean who cares. But in most hospitals if you look near a nurse station you’ll find them not hidden. But who gets freaked out by a trauma alert or a code blue at of all places a hospital? there’s no saving those types.
Having worked in a number of places it is more about expediency and scale. Code Blue was fine in the 1960s.. it was basically a tack on to other colors and it’s not too confusing. Now fast forward and there are different types of teams you want to call... respiratory, rapid response, ACLS, police, psych.. colors don’t scale well for this unless you want to go off of roygbiv.. and then you’ve lost.
Edit: there have been efforts to standardize. In the US sometimes you get consistency in a region.