Start by showing a picture of your window well, including some surroundings. And show another picture of the window in the well, all the way to the bottom.

I have seen many bad looking little curbs/walls made by homeowners out of various concrete edging/block materials that I suspect it is not the way to go. The idea of bulking up the window well with some type of "blocking" and then surrounding it with a raised bed made of more "blocking" sounds potentially, well ... not good. Justifying this creation by saying the area needs "definition" ... I'm not convinced. I think you need to a show the area in pictures and also show a plan of your proposed "wall" arrangement. You could be making a giant project that is only making things worse or is just not worth the effort (except for proper raising of grade which might need no wall or curb at all.) I'm mentioning this because to properly install such a structure is a good bit of work, not the mere stacking of blocks.

Soil will push the blocks inward unless secured in some way. They make metal window wells for this application.

The metal window well the is there now will be too low once the grade has been corrected. We plan to glue the bricks together with landscape block adhesive.

You can buy metal window wells in a variety of different heights. If the well is going to be very visible, sure, use retaining wall block. Just make sure it's designed for a retaining wall application (a lot of the big box store blocks, especially the 4" ones, are not). I'd rather see a block with a lip or a pin used vs one that just glues together.

But if you're really not going to see the window wells, call around to local building supply stores and get a metal one that's big enough, and put the time and money into awesome plantings. Lowes and Home Depot have limited size selections, but google for building supply near you and you'll find something.