As with any weapon in the game, there is an attacker and a target. In the case of the blue shell, the attacker is usually one of the last positioned racers and the target is always the 1st position racer. This is half the reason why the blue shell sucks.
Look at it from the perspective of the attacker: If you’re way behind, you want a weapon that will put you ahead. Your direct rivals must be taken out or bypassed before you can even think about going for the leaders. So if you’re in 7th place, who are currently your biggest rivals? 6th and 5th place. Who do you target when you fire a blue shell? The guy in way up in 1st place.
If you’re like me, you shoot the blue shell immediately to make room
in your inventory for something more useful (is there any sense in
holding on to it?). The 1st place kart that you’ve just taken out will
surely be up-and-running again before you get close to catching up with
him, and, even if you do catch up, whoever was in second place just took
over as the new first place.
Some help.
hit the jump for more