Attacks from affixes (bombs, arrows) do not inflict bleed stack from this mutation.
Weapons that already inherently inflict bleeding (notably, the Blood Sword and Bloodthirsty Shield ) will still be able to add another bleed stack via this mutation. So, for example, each slash of a Blood Sword will inflict one instance of bleed damage-over-time via the Blood Sword and one instance of Open Wounds bleed damage-over-time, for two bleed stacks per slash.