I've used blocks before and didn't like leaving the tops open as grass grows through and into the beds, and weeds are more difficult to remove from these small openings. Personally, I would tend to fill the block with sand and just cap the holes with concrete mix. I like the rebar idea, to help keep the blocks in place.
I have beds with no more than 8 inches of soil on a concrete pad, and lots of plants grow just fine. Last year I grew cucumbers, arugula, lettuce, very vigorous okra over 8 feet tall, Swiss Chard, kale, radishes, and more in the beds. I've read quite a few places where 6-8 inches is not adequate. In my experience, that depth is not optimal, but gives quite good results with lots of veggies. That said, if invasive roots are not a problem, then I don't see any compelling reason to block the bottom of the bed from the native soil.
In planning the bed, you may want to give some thought as to how you will manage the skirt area boardering the bed. Perhaps just mowing and using a line trimmer will suit you. I prefer my borders to be more resistant to weeds and grass. Currently am mostly using high quality landscape fabric covered with gravel. Mulch works o.k. but must be removed and replaced every year or two in order to remain effective.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.