Changing sprites from spritesheet on collision. What am I doing wrong?

I completely re-wrote your script, making it less complicated and fixing your problem:

 using UnityEngine;
 using System.Collections;
 
 public class block : MonoBehaviour {
     public Sprite[] blockColors; //Assign the length of the array in the inspector,
     //and drag the sprites you want it to change to into each element.
     public int currentSprite = -1; //The int that tells us which sprite in the array
     //we will use.
     public bool deleting; //Unity doesn't actually delete an object until the next
     //frame, so this will allow us to make sure the code doesn't break by having
     //currentSprite be longer than the array length.
     private int arrayLength;
 
     void Start() {
         arrayLength = blockColors.Length - 1;
     }
 
     void OnCollisionEnter2D() {
             currentSprite += 1; // adds 1 to the "currentSprite" integer.
 
         if (currentSprite > arrayLength) { //Deletes the object if currentSprite is greater than the
             //length of the blockColors array
             deleting = true;
             GameObject.Destroy (gameObject);
         }
         if (!deleting) {
             GetComponent<SpriteRenderer> ().sprite = blockColors [currentSprite]; //sets the SpriteRenderer's
             //Sprite to be the desired sprite in the blockColors array.
         }
     }
 }