I would paint the area above the mantle.

Since there is no break from the top of the fireplace to bottom and I'm speaking of the sides leading to the bottom of the fireplace; Should I paint along the sides as well. I don't like creating imaginary lines with paint.

Yes, I would continue with the paint.

I would also add more bold accessories into your shelving. It needs more contrast in my opinion. Or darker color of the background.

I would only paint the back of your built-ins to match walls. I did mine in the Dorian Gray BM that was used in my attached kitchen area. My accessories are much more of a traditional/French farm theme but the painted back drop works really well. You have a wonderful space! I don't think it is necessary to have to add to much color I really like your more modern/contemporary look and love all the neutral shades.

Is that blue above the can lights? Is that the color of the adjoining space? If it is staying, you might wallpaper the backs of the cabinets with a pattern that has that color in it.

The entire room and adjoining kitchen is agreeable gray... The light makes it blue on the photo but it looks gray throughout...

Not a fan of the wall paper look but I want something else for a pop... Fireplace color, add crown molding , something

I'm thinking of painting the top mantle to the trim sides for the added dimension.
Thanks all for the suggestion. This really helps.

Paint the niches darker as suggested it should do it

That looks nice @Tem... How did you do that ? Was it an app or full software?

I say go for it, it will add a nice contrast.