Why write in black and blue....unless those are one of your favorite colors? If not, go with what you love!
I started writing with different colors back in college. Just like the smelly highlighters, writing in various colors can help those who are visual learners, remember what they're written during class.
I've just kept it up since then because it became fun. Green is all I write in at work. Keeps my notes separate from anyone else's that's for sure. Plus it puts a smile on my face, even on the most chaotic of days!