Wear gloves during the sessions when you're just cruising around (especially early season or while on off-season vacations). You can use mountain biking gloves and I use paddling gloves from REI. Gloves also protect against sun damage. If I get a blister and I still want to sail, I use Second Skin to coat the area of the blister (trying to help make it waterproof) and then I tape it. I use a small patch of waterproof tape (because it comes off easier without pealing off healing skin) and then adhesive tape that is really sticky. I wrap it several times because it's not going to stay on for very long. As soon as the adhesive tape begins to come off, I go in and add another layer or two. When finished windsurfing, remove the tape gently. Let the blister air dry during the night.
My recommendation for calming down hot, sore windsurfers' hands, (without compromising much prized callouses!) is a product called 'Climb On'. It comes both as a solid bar, that you rub around your hands like you would with soap - great for straight off the water, or in a tube as a rich cream for more intensive repair (use overnight, as it is very greasy!) A 1oz bar lasts for ages (eg a year!) and all those that we have introduced to it are converts because it does actually work. It kept our soft, winter hands which we had not wrapped around a boom for months in perfect windsurfing condition for 2 weeks of intense sailing in Margarita, while all around us had their blisters strapped up with duct tape within days!
I have never had blisters since starting to use it 8 or so years ago, but if you do get blisters, pack them with 'Climb On' and it helps them to heal much more quickly. The cream is also good for calming sunburnt/windburnt and chafed skin.
I usually buy 'Climb On' on line, but it is available from some outdoor shops. The name clearly indicates that it is aimed at Rock Jocks with abraded fingertips, but I think that it is a fab product with equal application for we wind junkies!
I have found that in general, gloves don't really work for windsurfing. I do have a pair of full-fingered, shaped, ProLimit gloves with a neoprene back and leather palm which seem to be the only ones that do not pump up my forearms. I use them to ward off wind chill in winter rather than blister avoidance, however. Sometimes, gloves can cause blisters.
Interesting advice, and saw similar from a gymnast who suffer similar problem and use loads of hand moisturiser reckoning callouses give a pressure point and surrounding skin tears. I had 8 days strenuous sailing abroad in August and used cocoa butter as an after sun moisturiser all over my body so there was loads on my hands used to apply it. I was surprised I did not get blisters all week so maybe that is why. Just got some Climb on and does seem to accelerate healing.