This is actually one of my gripes about mario kart, not necessarily just the blue shell, but the limited availability while in first place. You can only defend so much, but it seems like the guy trailing you in second is pulling infinite red shells sometimes, lol.
Just drive off of the side before it hits you, you lose less time by having lakitu fish you back on track than taking a blue shell. Or slam on the brakes and hope you take others out around you