If it's blight, you need to pull up the plants. I'm so sorry... it will get into your soil if you don't. We deal with that a lot here (also PNW) and what we are told is that you have to put them in trash bags and actually throw them away. If you compost the plants, you'll spread blight all over your whole garden, and you won't be able to grow tomatoes, potatoes, or peppers for a few years. It's a fungus.
If it hasn't been rainy, another reason could be that you are watering in the evening, or watering the foliage rather than at ground level. Tomatoes Hate having wet leaves.
Now, if the black spot is at the end of the tomato, it could be blossom end rot which isn't so bad. If that is the case, the spot is just where the flower was at the tippy end of it, then you need to give the plants a good dose of liquid calcium. That is a foliar fertilizer, so you spray it on in the morning, giving the plants a chance to dry off before night.