Dehydration usually would result in hypotension (low blood pressure) and reflex tachycardia (rapid pulse) due to decreased circulating blood volume. But depending on several factors, including the sympathetic reflex, transient elevated blood pressure might be seen.
However, hypertension is not diagnosed based on a single value, but repeated elevated values over time. It is a different pathology than a temporary sympathetic reflex, and generally this is not fixed by rehydration.
Good hydration is an important part of good dietary practices that are the foundation for treatment of hypertension. However, diuretics (which reduce fluids) are also often used to treat hypertension. There is a complex fluid balance involved, and it depends on a lot of factors.
For any individual's application of these topics, see a physician.