blowing black smoke

It is normal for the exhaust pipe to be stained with soot. Where are you reading the intake air temp?

 

That is probably intake air at the intake manifold. Normal temps.

 

Having owned an '06 ISX, I'm also inclined to say EGR valve. The "unplugging the EGR valve trick" works like this: when the engine is dead cold, turn the key to the on position. Wait until everything cycles through and stops clicking. Turn the key back off. Then unplug the EGR valve. Be advised though, that as far as I know, the check engine light will be on as long as the EGR valve is unplugged, and many '05 and newer trucks will derate the engine with the EGR valve unplugged.