Thank you very much for taking the time to give me this feedback. It is much appreciated. I think your instinct is correct, because the graphic designer responsible for my cover said a similar thing (ie that the blurb, and indeed the synopsis I sent her, was inadequate, but once she read the opening pages she was completely gripped).
Book blurbs are an interesting problem for an author. My instinct is to withhold as much information as possible, because I like people to come to my stories cold (I try to avoid film trailers for the same reason - I like to know as little as possible beforehand). The trouble is, that's just too idealistic and impractical from a marketing perspective. I might well rethink the blurb for this novel, based on what you've said here.
One of my novels that was traditionally published, Spectre of Springwell Forest (a supernatural horror), had my original blurb entirely rewritten by the publisher. I hated the publisher's blurb because I thought it revealed far too much. What does that tell me? I'm a storyteller, not a marketer. At least, not of my own novels (apparently I can sell other people's books and films well enough).
This is an interesting topic. I might write an article on it at some point.
Again, thank you very much for taking the time to respond at length.