It is not just a calcium deficiency. It has to do with uneven watering as well, which affects how calcium is absorbed by the plant. You can address the Ca issue with a proper tomato or vegetable fertilizer that includes C as part of the formulation. You can also get a CA supplement like Rot Stop that will help. And keep the soil evenly moist, never allowing it to dry excessively or become overly wet.

I feel like a lot of this can be prevented by simply picking the right potting mix. What do you recommend for tomatoes? I know they like things more on the acidic side.

Calcium Deficiency can't be solved entirely by using the right soil. Even with the right soil, over the course of a year, the plant will use the calcium to create plant cells and equally important--fruit. To replenish calcium that gets used up in the potted plant, I personally use General Hydroponics CALiMAGic. My understanding is that calcium deficiencies aren't as much of a problem on non-potted plants.

But my big question:

For potted plants, how much of this stuff are people using?

I don't use it and have never had a need to treat for blossom end rot. Part of the deal is I focus on cherry tomatoes, which are overall not very prone to this affliction And I use a liquid fert routinely that provides all the Ca necessary for healthy growth. And I make sure my containers stay evenly moist - never too dry and not too wet.

My experiences are very similar to yours gardengal. I almost never have this problem with cherry tomatoes and can sometimes grow grape tomatoes (the medium size ones as I like to call them) without any problems. It's really only the large tomatoes I ever have problems with.