A blazer is judged at the shoulder. Everything else can be adjusted; that seam cannot.
The seam should sit where your shoulder ends. Past it and the blazer will collapse; short of it and the sleeve will pull every time you reach.
Ending at the wrist bone, with room for a shirt cuff underneath. Sleeves are the easiest alteration to make.
It should close without strain across the bust. If it pulls into an X, the size is wrong regardless of what the label says.
Cropped blazers end at the waist; classic cuts cover the seat. Both are correct — but pick the one that suits what you wear underneath.