Can a celiac plexus block help relieve nausea?

Sometimes. A celiac plexus block is usually used for intractable abdominal pain. There have been a couple of studies which have found alleviation of nausea, better control of bowels and better sleep in patients who received a celiac plexus block. Usually physicians turn to this block for pain. Talk to your doctor! for nausea alone there are several medications and patches which may be a better option.
Yes. The block will help with pain which may indirectly reduce nausea. It will also speed up intestinal motility which also will help.