All melee heroes have innate damage block by default, having a 50% chance to block 16 damage.
Kraken shell blocks 12/24/36/48 dmg for 100%.

so if you get hit while having kraken shell lvl 1. 50% chance to block 16 dmg. and remaining 50% for 12 dmg.

kraken shell feels underwhelming to skill early. suggesting to change it to 20/30/40/50 or 18/28/38/48.

my opinion.