A solution that comes to my mind, would be adding an image point to the position where the sword hits on the attack animation's last frames and sticking a small invisible sprite in it, which would then spawn particles on enemy collision.
Go to the hero's sprite editor and add an image point, set it in the position you want, in this case select the frame where the sword hits and set the image point to the sword's tip.
Then create a small sprite and make it invisible from its settings.
Every time you hit, make the hero spawn the invisible sprite to the image point you made earlier. If you want to be exact about the timing, compare the frame and select the one where the sword is supposed to touch the enemy. Start a timer, to destroy the invisible sprite some time after the hit is finished.
Check if the invisible sprite is in collision with the enemy and if it is, make it spawn the particles.
So basically the blood comes from the hero sprite, not from the enemy. I'm not sure how well this solution works and I haven't used particles myself before, but this is what came to my mind first.