I was shocked that this question had been asked in numerous forums all over the internet and NEVER ANSWERED. So I thought I'd answer this one based on this weekend's experience.

We similarly obtained a used Jenn Air single wall-mount electric convection oven: not sure of the model number (??198??) and were puzzled by the blue wire. We removed the back (a million screws) and were no further ahead- no wiring diagram, no connection diagram, no labelling. Jenn Air's website(s) were of no help whatsoever, and it was a weekend so no calls to Whirlpool's lovely "service centre" either.

We connected the neutral (white), ground, black and red, capped the blue and powered the oven, and got all functions EXCEPT the oven light. Testing indicated that there was no power to the blue wire relative to the neutral. Disconnecting and troubleshooting with an ohmmeter we found out that the only thing connected to the blue wire was the oven light, with the other connection of the oven light being to the neutral. So we merely tied the blue wire to the black, powered the oven back up and voila- all functions.

We have no idea why the oven light is powered by a different circuit supplied with its own wire- perhaps people wanted to be able to turn out the oven light with a separate switch.

Obviously the usual caveats apply: if you don't know what you're doing, hire a pro. Don't fry yourself or risk frying the equipment. For all we know, Jenn Air uses the blue wire for a variety of uses depending on model number. But that seems unlikely- and connecting up an electric oven is hardly rocket science. Cap off the blue wire and try it- there's not much risk in doing that. Missing functions? Then you need a wiring diagram or a little more electrical knowledge to figure out what's going on.

Best of luck-