jeans plus blazer or suit jacket: the "business mullet"
I don't think there's anything wrong with a sport jacket and jeans. Suit jacket? Yes. Faded/torn/light jeans? Yes, few of us can pull that off. But a nice pair of dark jeans with a coat that is obviously an odd piece? I think it looks good on younger guys...
...because I wear this combo often.
As with anything, a blazer is not a blazer is not a blazer; an SC is not an SC is not an SC; a pair of jeans...you get the idea. Success or failure with this combination really depends on the items being combined.
Rough, faded jeans with a finely woven, worsted blazer = fail. Dark jeans with a herringbone tweed, or a similarly sporty jacket, can be a decent combo. Generally speaking, the further from a suit jacket the odd jacket looks, the more likely it is to be successful with jeans. And the further from a pair of torn-up dad jeans a pair of jeans looks, the more likely it is to be successful with an odd jacket. Both items need to be drifting in the opposite directions of formality, meeting somewhere near the middle. (Though, to be honest, they're meeting somewhere closer to the jeans side of the spectrum. Getting a sufficiently rustic jacket into the mix is more often the challenge than getting a sufficiently dressy pair of jeans).
Glaringly bad examples of the "business mullet" tend to feature stark contrasts between the two items, for instance, an orphaned suit jacket -- often black, at that -- paired with faded, ripped jeans.