Fading in/out GameObject

I'm fairly new to coding, I'm still trying to develop that logic of thinking to help me create the solutions I'm wanting for games. Currently, I'm in Unity trying to create a 2D GameObject that's a wall hiding a secret door. I want that GameObject to fade out (about 90%) when the player GameObject triggers it, revealing the space behind and the hidden door.

So far, I've managed to figure out how to render the "secret wall" GO inactive on the trigger, so it disappears, but this doesn't produce the visual that I'm going for. As I said, I'm still working on developing that coder's way of thinking, so while I've done a lot of research to solve this problem, many of the results I don't readily understand.

Here's my code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SecretDoor1 : MonoBehaviour {

    void OnTriggerEnter2D (Collider2D SecretDoorTrig) {
         if (SecretDoorTrig.gameObject.tag == "Player") {
             GetComponent<SpriteRenderer> ().enabled = false;
         }
         else {
             GetComponent<SpriteRenderer> ().enabled = true;
         }
    }

    void OnTriggerExit2D (Collider2D SecretDoorTrig) {

         if (SecretDoorTrig.gameObject.tag == "Player") {

             GetComponent<SpriteRenderer> ().enabled = true;
         }
         else {
             GetComponent<SpriteRenderer> ().enabled = false;
         }
    }
}