Your mantle looks close to the color of your railings. I would try to stain the shelves as close to the railings as I could get. I am sure your railings and probably mantle too, have yellowed some over the years. Use oil based poly over the stain, so the yellowing will continue on the new stain. I would hope the mirror you are putting up will have a stained wood around it too. However, I would be tempted to put another set of the same shelving on the other side too, instead of a mirror. No, I would not paint the wall in the dining room, the dark blue. Just leave it like it is. The dining room is much darker already than your living room, and the blue would just make it darker still. I would leave the mud room alone too. It is light and bright, and looks like you have your stove there, etc. Keep it light and bright.

I would not paint the dining room wall dark.

I would paint the back wall (door wall) of the mud room dark blue.

I would not hang a framed mirror over the panel box, get a stretched canvas, long and narrow, not matching, but complimentary images for both sides of the fireplace, centered at eye level.

GreatbigCanvas.com is a great source for less expensive art.

What color couch and rug are you going with? If the new couch and rug are lighter in color, then I would consider painting the window wall and back wall of the couch in blue. I would leave the dining room as is.

I agree about putting art over the utility box and coordinating art on the other side.

I would paint all the walls in the mud room the same color as the dining room walls. The white door in the mud room looks off. Maybe because all of your other doors are wood. Is it the color of the trim? I might consider painting the door blue.

Agree the blue isn't the best choice. An accent wall should be a solid wall with no windows, FP, etc. No to floating shelves as you don't need more "stuff" to give visual clutter. Art on both sides of FP as suggested by @Lyn Nielson

Why a mirror? When hanging a mirror be mindful of what they reflect, as that becomes 'art' in the room. Your space is reading small in these pictures. Choosing such dark colors will only make the space feel smaller. You would be wise to consider Judy's advise. If you are loving the cave vibe go for it but consider upping your lighting to counter what the walls will be sucking out of your spaces.

Tiny plave mega clutter no mirror or shelves on that wall and honestly not the blue either . Accen walls best planned after all other things are done including homes for all the stuff . Then a wall where there is no interruption is best as an accent wall. In your case the FP was the accent wall.

I find the white baseboards quite jarring, especially sibce most of the trim is Craftsman wood tone