The most common mistake I see in blurbs — my own early ones included — is treating them as a summary. A blurb is not a synopsis. Nobody buys a book because they now understand its plot; they buy it because they want to know how the plot turns out, and the blurb's entire job is to make that question unbearable to leave unanswered.
The structure I use now, across horror, historical fiction, crime and romance alike, is roughly the same shape every time: a one-line hook that states the central tension, a short paragraph raising the stakes and naming who's caught in them, and — where it's earned — a comp-title or genre signal so a browsing reader instantly knows what shelf they're standing in front of. Measured Ground's blurb opens with "Rome has roads into every corner of Britannia. Now it is learning where its enemies hide" for exactly this reason — it states the entire conflict in two sentences, before a single character's name shows up.
That's the other discipline worth naming: hold off on character names as long as you can. A blurb with four proper nouns in the first two lines reads like homework. A reader doesn't need to know Kasir's name to be hooked by "every citizen trusts the numbers — until one man doesn't"; the name can wait a sentence or two, once the stakes have already done the work of making them care who this is.
Generic language is the other thing that quietly kills a blurb. "A thrilling journey of discovery" could be the back of almost any book ever published, which means it tells a reader nothing and persuades nobody. The test I use: could this exact sentence appear on a different book in a different genre without anyone noticing? If yes, cut it and replace it with something only your book could say.
One honest technique for checking your own blurb: the elevator test. Can you state what's actually at stake — not the plot, the stake — in a single sentence, out loud, without notes? If you can't, the blurb probably can't either, and that's usually a sign the manuscript's central question needs sharpening before the back cover will ever be able to sell it.
Once the words are right, getting them to actually sit correctly on a printed back cover — wrapping cleanly around an author photo, staying inside the safe margins, not fighting the barcode zone — is a separate, entirely mechanical problem, which is exactly what I built Spinework's back-cover layout to solve.