Kale could be use as a medicinal food source?
Oh really? I did not know that?
Kale was the most common green vegetable in Europe until the end of the Middle Ages.
What is the most common one now?
I am not sure, but kale plants have green and purple leaves and the central leaves don't form a head 
Can you eat it raw? 
I am pretty sure you can it is like any other green.
Is it considered a dark green leafy vegetable?
Kale
Kale () or leaf cabbage is one of certain cultivars of cabbage ("Brassica oleracea") grown for their edible leaves, although some are used as ornamentals. Kale plants have green or purple leaves, and the central leaves do not form a head (as with headed cabbage). Kales are considered to be closer to wild cabbage than most of the many domesticated forms of "Brassica oleracea".
Etymology.
"Kale" originates from Northern Middle English "cale" (compare Scots "kail
Palmer; this includes plants like cabbage, broccoli, cauliflower, brussel sprouts, kale, collards, various mustards, radishes, rutabagas, and turnips. Potatoes, beets, carrots, spinach, and lettuce also grow very well here. The Vanderweele farm is the biggest commercial vegetable farm in Palmer.
Palmer holds the world records for kale, kohlrabi, rutabaga, romanesco broccoli, turnip, and the green and purple cabbage. The record for the cabbage was set in 2012 at the Alaska State Fair with a weight of