For a Blitzle it is better to evolve her. This is because Zebstrika is meant to be fast and hit hard with physical attacks. But it is rather frail. Items like the Eviolite that increase the defences are better on Pokemon with fairly high Defences already. Gligar, Porygon2, and Chansey are particularly notable users of it. Zebstrika/Blitzle is meant to dish out as much damage as it can before fainting, and is simply not meant to take a load of hits.
Also, I'm pretty sure the Zebstrika has better defences than Blitzle with an Eviolite anyway (Blitzle has base Defence and Sp. Defence of 32, and Zebstrika has 63. Since the Eviolite increases the defences by 50%, Zebstrika still has better defences).
