Now, keep in mind that I’m possibly in a class of few, but, personally, I never read back cover blurbs. Especially if the book I'm considering is by an author I already know and love. For me, it's like reading a spoiler.
If I'm contemplating purchasing a book by an author unknown to me, I’ll read the first page, maybe a tad more, to see if the writing style connects with me and the story/character(s) grab me. If so, I buy it. If not, it goes back on the shelf.
The lesson here is that, yes, your cover is the first thing readers will see and that's important for sure. As is the blurb you put on the outside of your book. But never forget how crucial that first page of the story is as well. Remember that a large majority of readers won't give you much more than that first page to pull them in.
So, always do you best, but be sure to put your best work where even an unlikely reader can see it. Do that and they might not be an unlikely reader going forward.