I noticed that sometimes when I come to GOG, it says Blood has been updated. But I can't find anything about what actually has been changed. Is there a place where I can find this information?

If I recall, it was a better soundtrack. I think it was around the time it was on promo.

Also, the Type-O Negative video

Great, thanks to both of you.

Is there an obvious place where updates are listed that I'm just missing? I expected some sort of list with info or something, but have had no luck.

They are just download links listed in your game collection. When you click on the Blood box on your game shelf, it will show the download links for all the content you have access to.

Game downloads
Game installer

Bonus content downloads
manuals (updated)
soundtrack (updated)
'Love You to Death' music video (updated)

When there is an update it will just be part of the list, and generally have a red update notice next to it.

Other than that, it's just trawling the news pages when the update sign appears and seeing what comes up.