It's not about positive/negative. Those have specific meanings in research, and it's not "good vs. bad" like in colloquial speech.

Placebo/nocebo are both positives, but the difference is about whether the imagined effect is beneficial or adverse. Imagining a beneficial effect is a positive, and imagining an adverse effect is also a positive.

"Negative" would imply they're imagining that something isn't there. For instance, if clinicians could verify that a physiological change took place, but due to the subject's expectations they don't notice any change in symptoms. Like, "I thought it was a placebo so I don't feel any better." I don't think there's a word for that because it's not typically how trials are designed.