Also, we’re assuming that Amazon classifies Psycho as a horror movie and they famously didn’t use fake blood at all, they famously used chocolate syrup.
There are, of course, a few issues with this particular breakdown of the numbers.
The amount of fake blood used in older movies was much less than it is now, so those additions would either be too minor, or their being accounted for with the current averages.
There’s also the assumption that every horror movie has been put on DVD.