Concur with amaranth, sweet potato leaves & New Zealand spinach.
NZ spinach can be very slow to germinate unless pre-soaked, and the tiny seedlings can easily be mistaken for weeds. The plants are not only heat tolerant, they will take a few frosts. I agree with the blanching recommendation.
Egyptian spinach (or molokhia) is another good hot weather green, it is on the left side of the photo above. An upright, heavily-branched bush. The leaves are mucilaginous (like okra) and somewhat bland, but it is good with fish, and the dried powdered leaves can be used as a soup thickener. The young leaves are mint-like in appearance, deep green, tender, and can be used raw in salads.
My favorite hot weather green, though, is water spinach. It grows across the ground very much like miniature sweet potato vines (it is in the same family). The tender vine tips & the first few leaves are harvested. The more it is cut back, the more branches form. Filipinos call it Kang Kong, and I like it cooked & seasoned with soy sauce, vinegar, and garlic powder. The plants thrive in wet soil... but in some of the warmer states, it is classified as a noxious weed, because it is a water-borne invasive. Seeds are hard to come by, but shoots are often sold in Asian markets, and those can be rooted.
The last hot weather green I could recommend is Moringa oleifera - which is actually a small tree. The tiny leaflets are eaten, and are very nutritious. I grow them each year from seed & transplant them into the garden; but it can also be grown in pots & brought indoors for the winter, or left outside in frost-free zones. It will not produce as heavily as any of the greens above, but it is worth growing for its health benefits. The trees need well-drained soil, and will languish or die if waterlogged. If grown as an annual, the trees can be spaced much more closely than in the photo.
