Blend Cubemaps in reflection probe

I am trying to make a reflection probe render once every second, and blend between the old render and the new one. See how it looks like in Zelda BOTW: https://twitter.com/brainchildlight/status/937439281890467840

.

I am currently only able to render once every second, but not blend between the old and new cubemap. It instantly swaps to the new one. I found the ReflectionProbe.BlendCubemap function but have had no success with it. Most attempts I make crash Unity, and since there are a grand total of zero examples of it being used online I have a hard time even figuring out if it is the way to go.

I am hoping not to have to rely on blending the cubemaps inside the shader since I want this to work on all objects with all kinds of materials, not just certain ones which are blend-approved...

Here's some of the code I've tried, which in my mind should probably work. All it does is render a new reflection every second with no blending. But as I said there isn't really that much information available about this stuff so I have a hard time to figure out what I am doing wrong.

 public class PlayerReflectionProbe : MonoBehaviour
     {
         private float timer;
         public float updateTime = 1f;
         private ReflectionProbe thisProbe;
         public Texture oldCubemap;
         public Texture newCubemap;
         private RenderTexture finalCubemap;
         public float blend = 0.5f;
         public RenderTexture render;
 
         void Awake()
         {
             // Set up reflection probe component.
             if (thisProbe == null)
             {
                 thisProbe = GetComponent<ReflectionProbe>();
             }
             // Set up final texture diplayed by render probe.
             render = new RenderTexture(128, 128, 0);
             render.dimension = UnityEngine.Rendering.TextureDimension.Cube;
             render.useMipMap = true;
             thisProbe.customBakedTexture = render;
         }
 
         // Update is called once per frame
         void Update()
         {
             // Blend between old and new.
             ReflectionProbe.BlendCubemap(oldCubemap, newCubemap, blend, render);
 
             // Assign final render to probe ???
             thisProbe.customBakedTexture = render;
 
             // Update blend
             blend -= Time.deltaTime;
             if (blend <= 0f)
                 blend = 1f;
 
             // Render new once every second, and save old render.
             timer += Time.deltaTime;
             if (timer >= updateTime)
             {
                 oldCubemap = thisProbe.texture;
 
                 thisProbe.RenderProbe();
                 newCubemap = thisProbe.texture;
 
                 timer = 0f;
             }
         }
     }

Any help at all is greatly appreciated