I should add: I plan on brewing some worm tea as it may add phosphorous safely in a faster and safe manner than some other means. I'm not positive.

I am not so sure it is actually a phosphorus deficiency.

Are the purple splotches on top of the leaf? On the bottom? All the way through the leaf surface? Generally splotchy-looking Swiss chard leaves have a fungal infection like cersospora leaf spot. I tend to ignore most things until they go away on their own. It works for me.

Usually when you have a phosphorus deficiency issue, it shows up as purple on the undersides of leaves or on stems. Often it isn't even really a phosphorus deficiency. Instead, it is cold temperatures impeding the uptake of the phosphorus, and it corrects itself on new leaves that form after the temperatures warm up. Unless you've had a soil test and know for sure that your soil is deficient in phosphorus, I wouldn't go adding phosphorus to the soil. Too much phosphorus can be as damaging as too little. Any time that a person starts picking and choosing and adding this nutrient or that nutrient to the soil, if you add too much of one thing, it can affect the way that the plants take up and use (or fail to take up and fail to use) the other nutrients. That is why you need to be careful when cherry-picking a single nutrient to add without the benefit of a soil test telling you for sure that you need it.

Worm tea won't hurt, but might not help. If you feed the plants and see no change in their condition, then it is likely that the plants have a fungal infection.

If it looks like a fungal infection and if you used seed from the same packet that you had last year, then I'd throw away that seed because it appears it may carry the infection.

I am a very relaxed gardener and don't stress over leaf splotches. I cut off the splotched leaf and throw it away. If the new leaves that come out have the splotches, I spray with a fungicide. Or , it warms up and the splotches go away on their own, which you will notice as an absence of splotches on new growth.

If you've had decent rainfall the last few weeks, I'd be more inclined to think it is fungal in nature, or the repeated cold nights are interfering with uptake of nutrients in the soil.

It it has been really dry and you have been watering, be sure to water the ground but not the foliage.

I'll snap a picture of the next occurrence. It's really strange and doesn't fit descriptions or pictures I can find. It's inconsistent, affects all the plants grown in that area and occurred last year when it was exceptionally hot with consistency.

Watering it inappropriately could be a cause. I knew I'd regret being too lazy to snap a photo.