As Death Blossom is a Dual Attack, the area-effect damage from this skill is applied twice. The area-effect is considered direct damage rather than an attack, so it cannot miss or be blocked. Area damage is only triggered when an attack strikes, so anything that causes your attacks against the target to miss will also negate it. Note also that you'll only do one packet of area damage if your first hit is sufficient to kill your target.
The radius of the area damage is centered around the target.
The area damage does not affect the primary target.