Athena the covers are so important and as an author, although we don't have enormous control over what is suggested, after four novels, I'm beginning to get the hang of what's required. My third novel, DEITY, is about the vulnerability of teenagers and explores the ease with which a so-minded individual can push them towards self-harm or suicide. I quite liked the cover when shown to me, it was moody and mysterious, but I've since realised that it did not shout "Crime Thriller" at customers. With The Unquiet Grave, I was able to mention this and was rewarded with a cover that corrected that omission as well as intriguing the reader.
I know the blurb is important, though I personally don't read more than two lines. I am now in a position to write my own blurbs thankfully but at my previous publisher I didn't have that power. Having strived for a year or more to conjure up half a dozen seriously good shocks I found it galling to have three of them given away on the back cover. Probably why I don't read them.