The Master Blaster won't help with water spots if you are working outside and there is sun out (hot panels). I am assuming you are - as that is where you usually get water spots.
I can offer a few suggestions, as like you - I have a black car and wash it outside. First - buy some type of water spot remover. I use Duragloss, but CarPro seems to be in vogue. Second, the real trick is to keep the car wet and rinse it with DI water.
Your not wetting panels necessarily to "keep them wet" - your wetting panels to keep them cool. Try to keep that in mind. Also, concentrate on keeping the horizontal panels cool - as these will get hottest from the sun and pool water. Work fast - but not so fast you forget to keep the panels wet.
Lastly, you need to rinse well with DI water. This is key. After you rinse with DI water, assuming you did everything else right, it won't matter how you dry. In fact, this is when I let my car bake in the sun and clean my buckets and what not. Then, use the Master Blaster or a towel.
Another thing that helps is to wash your car at the crack of dawn before the sun is all the way up and really hot. If you live in the desert or something - I'm not sure if that helps much.
Clean your wheels first - but don't wet the rest of the car until you are ready to clean it. The water spot problem is caused by tap water baking dry on panels - so try to avoid this at all costs. Also, you can clean your windows first with glass cleaner if you have a lot of glass. This is one less thing you will have to do during the 2BM - making it even faster - so less time the car will have to sit with tap water on it.