Building a rectangular pond and raising it up enough that you *need* a continuous coping have made your goal of a natural look really, really hard to achieve. If you do a bluestone coping, or square off the stone you have pictured, you're just reinforcing the shape and the formality. At the very least, I would sawcut notches in the block and set a few boulders in to break up the coping, sort of like we did in this project:
Beyond that it's going to be a matter of how you landscape around it. If you integrate boulders into the coping like I mentioned and then work similar boulders into the plantings, you'll be able to visually distort the perfectly rectangular look of your pond if done well.
Counting rows of block it looks like you're at about 36" depth. Is your yard fenced or is there going to be a fence around where this feature is going? Depending on where you live, you'll probably be subject to the same barrier code requirements as a pool.
Love the screened and stone retreat, but not sure how you can get the "pond" which looks more like a basement, to conform to that vibe. Needs natural rock but that may not fit in unless you lower the top to ground level. Then a black liner will make it look more like a deep pool. Native plant materials for landscaping will help but the two PVC pipes sticking out will ruin anything you do that doesn't camouflage them.
Have you consulted a book or expert?. If you are going to have plants and fish you need to have different depths with "shelves" to place plants. Right now it seem to be more of am in-ground pool, than a pond. You could create a waterfall or spillway. Did you have a plan in mind or just start digging? You should decide what to do before continuing work on it.
Decoenthuiaste The 4" pipe is for a skimmer that will attach to the side wall which connects to a T that is part of a 4" bottom drain that run under ground to the concrete shed under the screen porch. The 2" pipe is the return for the water coming out of a pre filter, filter and UV. I was think waterfall or potentially an urn on its side that would let the water pour back into the pond. The bottom of the pond do have a shelf that is about 2' high and 3x4' across (a boulder I could not move) that will be used for some plant life. I plan to use a liner over the center block that is raised above the ground 3" to stop ground water from running back into the pond (fertilizer and such). Trying to figure out how or what to cap the liner with. Blue stone would be my final option. Thought of using a cement saw to cut a square edge into rock that would sit on the liner over the center block. Your thoughts :)
Seems very complex - are you raising talipia in there? My "pond" is a 5x8x3 oval hole with liner and a fountain pump. Fish and frogs do very well in it but I'm not raising specialty fish. Maybe you're over-engineering a simple fish pond, but it can still work. I'd just be concerned to hide all the mechanicals. Blue stone is nice and don't forget the plants.
Do a web search on Japanese onsen (outdoor hot spring baths) often these have raised sides but they tend to really make an effort to blend them in to nature and you may get some good ideas. Beware - this search may find rather graphic images of people bathing so don't do it if you will be offended by co-ed outdoor bathing.
While we are never fans of the over use and abuse of huge amounts of rocks, this would be the exception. Start with boulders at least in the corners, then continue outside in order to camouflage the box.
Revolutionary Gardens