You can avoid this by simply rolling toward the blood as it is flying toward you.

If you are far out of melee range, he will summon a magic rune above him, reach into it, then quickly fling a wave of blood at you.

If he reaches behind and charges up the spell the explosion will be bigger but focused directly in front of him, making it easy to side step and strike him a few time. Wait a beat then dodge roll backwards to avoid the damage. If he claws the area in front of him a small explosion will appear after a delay.If you battled Mohg in the Subterranean Shunning Grounds you will already be familiar with this phase of the battle, but if not here is a little refresher:

It is located at one of the lowest points of the sewer, in the swampy area with the giant lobsters.

Before doing this boss fight it is highly recommended that you complete the Subterranean Shunning-Grounds (aka Leyndell Sewers) or at least go there and find Mohg's Shackle, an item that temporarily stuns Mohg and makes the first phase of this fight infinitely easier.

Download subterranean shunning grounds