When I buy a Blu-Ray I'll download the digital copy that comes with the movie from iTunes. If for any reason I lose the file would I be able to download the movie again from iTunes without the digital copy disc?
I don't think the OP is buying from iTunes, but he is looking to download the digital copy of the movie which comes when you purchase certain hard copy movies in a brick and mortar store. They offered this with some versions of The Dark Knight last year.
To the OP, for what it is worth, when I got TDK, I was able to download my digital copy to each of my desktop & laptop without issue. I don't know if that's the norm or not as it was the only content I have purchased so far which included a digital copy with purchase.