Add a blend tree child in script

I'm trying to make a script that will take in a list of animations and a blend tree and add the animations to the blend tree. But Unity doesn't recognise the Blend Tree namespace, but I don't know how else I would access the blend tree. I have a feeling I'll need to go through the animator, but if anyone can point me in a better way, I'd appreciate it.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class AnimationSwitcher : MonoBehaviour
 {
     public List<Motion> animations;
     public BlendTree blendTree;
 
 
     public void addMotions()
     {
         for (int i = 0; i < animations.Count; i++)
         {
             blendTree.AddChild(animations[i]);
         }
     }
 }