You do realise that anyone can be called "Blondie", right? Just because someone says "Hey Blondie" doesn't mean they came up with that line....heck I'm called Blondie sometimes...

^ Yeah

I'm always called Blondie. That doesn't mean my friends and family stole the line. Anybody can be called Blondie, just as a brunet could be called Brownie.

Blondie is just a simple nickname.

OR stereotypically, some people see blondes as stupid(no idea why) so some people use 'blonde' as an insult towards others:P its like calling someone with Ginger hair 'ginge', So, i really wouldnt see this as a direct reference, its more likely eugine is jsut being snidy about the fact Rapunzel is blonde, and he does use it as, well, not an insult, but more of a catty sort of thing to say e.g when hes trying to get Rapunzel to go home, he calls her 'blondie' to undermine her, even though she was very clear about her name:P