What's the soil mix?

And have you used any fertilizer? If so, what, when and how much?

The soil is a mixture of equal parts organic humus and composted manure, a comparatively smaller amount of peat moss, and then a couple 1/2 cu ft bags of sand. Bed is 8x4.

Everything was thrown in and mixed well last October. My wife and I planted some rows of seeds just to see what we could grow out of it even thought it was late into the season. As expected, very little growth. After about a month I covered it with black plastic until the start of this season. I remixed the top foot or so around and planted this year's seeds. The direct sown seedlings weren't growing well compared to the seedlings I bought at Lowe's, so I bought some composted manure (20% composted manure) and spread just enough around everything to barely cover the existing surface hoping that would help. Now I'm kinda stuck and am not sure what else to do, if anything.

Good job setting the table, but you forgot the food. You need fertilizer. For the fastest fix, get some fish-based stuff and drench with a strong mix. Then look around for an organic fertilizer and start using it according to label directions. You will see a huge difference. All that organic matter you have will help hold onto nutrients and become a trustworthy soil, but first it needs an initial food supply. You'll need less as time goes on.

hmmm, looks like a disease to me. Maybe early blight exacerbated by under fertilization?

and then a couple 1/2 cu ft bags of sand

Wonder what your pH is now? An alkaline pH can tie up nutrients causing stunted growth and chlorosis.

I'd yank those potatoes, apply a soluble fertilizer to the onions and lettuce and then get a soil test. Try the potatoes again next year, lil earlier maybe.

It is just blight. Pull them and dispose of them, and plant something other than solanaceae there.