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.