Isn’t it when ui/ux uses deceptive practices to confuse the user into doing something?
Yes. It’s when the UI leads YOU to do the thing you don’t want. So unless the banner telling you that the site uses cookies is doing something to make you accept them when you don’t want to accept them (such as by not having a button to not accept them visible) having cookies itself isn’t a dark pattern.
Most sites I’ve visited since they had to disclose their use of cookies have clearly visible “accept all” and “reject all” buttons, along with a “more information” button that often lets you configure what cookies you want and what you don’t.