I think a lot of people are underestimating how fucking good blue belts can be. I think that was the main reason I felt I didn't deserve my blue when I recieved it. I was comparing myself to people who had been blues for years, and dominated in every comp they entered (ie- they were pretty much purple standard)
One guy at my school only recently recieved his purple and he's been training for like 8 years.
He's had some pretty serious back injuries, and has taken a real intellectual approach to BJJ.
Even my coach said to ask him about certain positions.
I've taught a few classes, and I was really worried about it, but apparently, according to my coach and the guys I was teaching it went really well.
I think a lot of blues out there are perfectly fine teaching complete newbs. Obviously you're better training with a black, but if you're just starting out, a good blue should be sufficent.
Oh, like someone else said, I'm talking about a high level blue, Not someone who's just recieved it.
Standards between schools vary greatly, sop some blues would be good purples by other place's standards.