How large are the containers, what container mix are you using, and what are you feeding them?

@edweather 5 gallon containers. I mixed garden soil, potting mix(peat moss, pine bark, perlite), composted manure from sheep and cows. Added small amounts of coffee grounds, wood ash and crushed egg shells at planting. I fertilize every week with manure tea. When fruits started setting I gave them a calcium based fertilizer - right now I have around 50 tomatoes set varying in size out of 20 containers and not a single BER case. The 2nd truss is setting now, 3rd and 4th are flowering. I'm not very experienced to recognize their needs yet but I guess some nutrients are missing. Perhaps the plant is too exhausted and needs more hard fertilizing?

@edweather I'd definitely get a proper soil mix without dirt next time. This is my first time and I wanted something cheap to see if it's worth growing in pots.

Today I did a check on one container which I haven't watered for 4-5 days. Temps been around 80F. The first inch was completely dry but as soon as I started going deeper the soil was well hydrated and bottom being slightly wet. If I water, the top layer of dry soil gets saturated quickly and water escapes in no time. That makes me think the water is retained longer in my case and should be watering less often.

Google phosphorus deficiency in tomato leaves. You need to give them a liquid fertilizer right away. Preferably one low in nitrogen, but higher in phos. And potassiu. Fish emulsion should work.

Fish emulsion is mostly nitrogen. That won't help.

I couldn't find the fertilizer brands you mentioned in the store so I just got a balanced water-soluble 18-18-18 with micronutrients. I gave it at 1/3 strength via a drip irrigation. This morning the purple spots were still there. How long does it usually take for plants to recover the missing nutrients?

You will see "recovery" in the new growth, not the older leaves already affected.

Can you post a photo of the youngest growth?

@gorbelly I'm actually surprised that after few days of heavy storms (thanks god shade cloth saved them from the hail) and very little fertilizing most seem to be recovering and look better now.

They look fine. Probably just had a little extra need for feeding since they're getting loaded with fruit. As for fertilizing, light and more frequent is much safer and more effective than heavy and less frequent, especially for container plants, so you did the right thing.