In a later project, we played with the idea of using an asm.js-compiled Webkit to render SVG (with embedded HTML) into a canvas due to mixed browser support.
Long story short, it's not inconceivable.
Jokes aside, even with rendering to canvas, ad can be blocked with image recognition. Long time ago I used Sikuli Script for UI testing automation, Sikuli recognized parts of screen and worked very fast; combine it with an app like CinemaDrape and voila.
[0] (loud noises warning) https://youtu.be/469zNXTCHdk?t=10
While many players, despite deeply nested html, still point directly to where the content is hosted (ex, a file on openload.co), an increasing number take your mentioned approach and load the content as a blob so folks won't bypass their ads.