javascript can't find blob

So I know that blobs are only available in the tab they were created in, but somehow I get an error message that the blob couldn't been found. It doesn't matter where I try it.

For example YouTube videos are blobs, I can get the object URL of it with

document.getElementsByTagName('video')[0].currentSrc /// => 'blob:http://youtube.com/asddasd-asdasd-asdasd-asddsa'

now if I try to get the blob with

fetch(url)

I get an error that the blob couldn't been found.