There is debate about what is necessary. However, the vast majority of the studies in this area specifically looked at grip strength and grip strengthening exercises. Other isometric exercises (and exercise in general) have support for lowering blood pressure, but perhaps with less impact. Highest chance for success seems to be grip strength exercises. It's also something you can easily do while watching TV, which might improve adherence.
Is grip strength really a proxy for doing resistance training in general (people who do full workouts having stronger grip)? Just speculation on my part due to forearms not really being a major muscle group.
Some of the studies seem to control for this. Some have looked specifically at isometric vs eccentric, etc and found the isometric grip strength exercises to be more correlated.
On a side note, there have been studies about grip strength and lifespan, including on small children who have never strength trained at all. Higher grip strength had a fairly strong correlation with longer lifespan.
