"/testfor @e[type=PrimedTnt]" - Bugs.

WAI
It only triggers it when it detects a PrimedTNT

You failed your redstone clock : The redstone between your comparator and your command block will switch between power 15 and power 2, put your command block 2 blocks further from the comparator.

I've put redstone over the command block too. So it will trigger the command block.

I won't work because the command block is always powered, just put a redstone lamp instead of the command block, you will see that the lamp don't blink. So just put the command block 3 block away from the comparator.