Why would a key get stuck in the on position in a 1990 Chevy S10 Blazer?

It's most likely the spring cap on the lock plug that keeps the lock wafers and springs in place. That's what allows the lock to work properly. (It's much more common for the cap to spring in the locked position, making it impossible to turn to the On position.) When the cap springs while the vehicle is running, your only real option is to get to a dealer or locksmith who knows how to remove the steering wheel, disassemble the steering column down to the lock cylinder and remove/replace it without making a royal and EXPENSIVE mess out of the deal. Don't try to stop the engine; you won't be able to restart it, then you will need to tow the vehicle as well - adding another Benjamin to the bill!Replacement lock cylinders are readily available and only cost about $25 if that much plus the labor to remove/replace it, about 30 minutes.