It can be both of them, like a coin has two sides. Blinking is considered to be a necessary action, because it can not only moisturize the eyes but also keep out foreign objects from entering your eyes. The eyelids are muscles that can move fast enough and not interfere with your sight. It can be define as voluntary and involuntary muscles on your eyes. One reason is you can intentionally control eye blinking and stop it, but your eyes would get tired within one minute. And your eyes would become dry out soon. Hence, the other reason is it would blink again by itself to rewet your eyes and relieve fatigue.
The blinking eyes are in the voluntary state. When we feel uncomfortable at the eyes, we will blink the eyes unconsciously. However, too much time a second to blink the eyes may increase our eye pressure which will make your eye nerves get intense. It will be bad for the eyes. You'd better go to the hospital to have a look.
Well, as far as I know, eye blinking is one of the most common and voluntary reactions our body will generate. Here I must remind you that, it is originally unconscious, however, our mind could control it, asking our eyelips to blink or stay put. Actually, for most of the times, we blink unconsciously because our eyes need to be moist by doing this. Based on this point, it is voluntary.