How to trigger particle effect

Untick the loop property in the particle settings and use the Emit() method

http://docs.unity3d.com/ScriptReference/ParticleSystem.Emit.html

Actual implementation depends on what kind of effect you have, but on your case you will most likely look like this:

 MyParticleSystem.Emit(50);

Play a little bit with the particle system in the inspector till you have the effect you want and you only need to pass the number of particles to spawn as argument.

     [SerializeField]
     protected GameObject thisobjectBlood;
     private void OnTriggerEnter2D(Collider2D colidedObj)
     {
         if (colidedObj != null) //check for tags or whatever else that can hit this object
         {
             //now if this is a thing that can hit your object:
             //if for this object:
             Instantiate(thisobjectBlood, thisTransform.position, thisTransform.rotation);
             //if(outOfHealth) Destroy(gameObject); //and for example instantiate explosion or whatever
             //you can do the same for collided object here or in the object itself depending if can be destroyed by this object
         }
     }