Is this Blossom End Rot?

[jhkokst]

First time poster here.

My girlfriend and I decided to pick up some tomato plants earlier in the year. Our goal was not a bountiful harvest, but more of an experiment to see if we could grow tomatoes that taste better than the bland stuff in grocery stores. We live in the high country of CO, so our growing season is about 1 month. We decided to grow them indoors through early June, and began to let them outside during the day for the past month or so. They still come in at night. We have 1 Supersweet 100, 1 Galina, and 1 Bucks County Hybrid. All are in 3 gallon pots (from my reading i know this is too small, but this is a test year). Our cherries set fruit in late May, and we picked our first this past week. Our Buck's county still has a way to go.

We have begun to notice some "rot" on the bottom of some of Buck's County. I am assuming this is blossom end rot. In the attached photo you can see what I am referring to with the tomato I am holding, as well as the one that that is blushing at the top of the photo. What's the quickest solution to fixing blossom end rot?

Next year will be different. I plan on starting some heirlooms from seed and putting them in more appropriately sized pots. This forum has really opened my eyes to all of the varieties one can grow, looking forward to it!