So a few things:
If I remember correctly BlazePose does not fair well on iOS devices in case you were running on one of those you will need to use the TensorFlow.js version to get support there as shown below.
Looking at your code I see in your HTML you import:
<script src="https://cdn.jsdelivr.net/npm/@mediapipe/pose"></script>
But in our TFJS documentation it asks to import:
<script src="https://cdn.jsdelivr.net/npm/@tensorflow-models/pose-detection"></script>
Can you try following this example exactly and check what comes back:

  
      

      github.com
  

  
    

tfjs-models/pose-detection/src/blazepose_tfjs at master · tensorflow/tfjs-models 3

  master/pose-detection/src/blazepose_tfjs

  Pretrained models for TensorFlow.js. Contribute to tensorflow/tfjs-models development by creating an account on GitHub.

  

  
    
    
  

  


Also when trying to run your example on my desktop it says video not ready and doesnt do anything so there could be an issue with your video starting