I’ve definitely had (good) blends were the components were taken to significantly different roast levels.
AFIK generally the components in blends are roasted separately for added control. Different beans behave differently in roasting so coffee that is blended then roasted will generally not be consistent anyway.
The separation lets a roaster take components to different levels to compliment each, eg. Roast a component with really good body but harsh flavor relatively dark to reduce the perceived bitterness, or keep a component you’re adding for fruity flavors or acidity light so you don’t suppress it’s desirable properties.
The former (harsh but big-bodied) thing is a common trick for Espresso in particular, a lot of really big-bodied beans tend to taste harsh, and that can be reduced with darker roasts without killing thr body. Robusta/Canephora (rather than Arabica) beans especially tend to be big bodied and highly caffeinated and hardy to grow…and have a major burning rubber note to their flavor. Good espresso blends often add some to improve mouth feel…but also cheap coffee products tend to use it, most instant or coffee flavoring starts as Robusta.
Single origin doesn’t automatically mean good coffee, but roasters who bother to source and label a single origin (which can sometimes be as specific as a farm, or broad as a country) will tend to be more mindful of that particular beans’ flavor. Also, smaller fancier roasters will generally sell fresher coffee. Beans that have been sitting roasted in a grocery chain’s “nonperishable” supply chain for months will essentially always be stale, and as soon as you get a taste for coffees that aren’t, you are cursed with that knowledge.
Single origins (and “weird” drying processes other than fully washed) will also tend to have way more character than “just coffee” which is fun and interesting but not always desirable. You can build really delicious (and consistent) coffee with blends in ways that might not be achieveable with a single bean.