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?
It is considered to be closer to wild cabbage then most domesticated forms