The grout was probably mixed that color and not painted.

I'm guessing that the rock façade is attached to a concrete block interior.

The vents are probably a passive heat distribution system since you can't find a switch. Cold air enters from lower vents and exits from the top when warmed.

Probably going to need to remove the rock façade to get wiring in there for the TV. May not be as expensive as you think.

Thanks for the response JRB451. I found a side view of the fireplace and I'm wondering if this might shed any more light on the grout/mortar situation.
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The idea of dismantling this totally freaks my husband out.

It would freak me out too. That's a ton of work, literally. Paint it a light colour, you may like it.

You could run a hose or something flexible thru the ducts to check if a fan is in there. If you put fans in have em suck in the top and blow out the bottom.

Looks like a heatalator - steel firebox inside masonry fireplace. area around firebox is hollow - usually the facing masonry (stonework that you see) is structural and supports the masonry above it which will be dimensional stone faced over a block core. Most often those vents allow for passive (convection) heating of the room when a fire is built. If you want to change the color of the grout, use a stain or colored thinset tile mortar painted over the current grout lines; this will be painstaking work to do neatly and nicely, but no where near the mess of a tearout and rebuild.

Thanks for the responses. What would I use to stain the grout?

I had a Heatilator fireplace in a house I had. That setup in the photo exactly what I had. It's a prefab unit with a stone façade. There are two ducts running to the vents and they just distribute radiant heat. There are no fans.

Not by any means a pro but a pretty good self taught DIY'er and have tried some pretty good off the wall ideas that have worked well. First off, I don't think you can stain your grout lighter over a darker stain. I know you can go darker but it sounds like that isn't what you want and it already looks dark. It is the living room side with the stone you are trying to change the grout color right? It looks like you have framed out the kitchen wall and eliminating the granite side (or perhaps that was for demo only?). I would mix a lighter color of grout that compliments the stone and to your liking and mix the grout to a little thinner consistency. Using a grout bag, pipe the grout out between the rocks and flatten with a paint brush, pushing the grout around the sides of the rock and covering the darker grout. It appears your grout is set pretty deep around the stone so you would have room to work it in and cover the darker grout without making it too shallow. Not sure but wonder if you could add a little bonding agent to your grout to help it grab. If the existing grout has been sealed, might want to use a metal brush and score it up a bit before adding another layer of grout.

Now to the tv issue.....I recently did a manufactured stone fireplace in my master bedroom and constructed a recessed box to install a motorized tv mount above the mantel/shelf. Was a real pain to do. After I was hooking up the tv, found that it fit nicely just on the shelf and there was no need for the recessed box. Would that work for you?

Are you framing in the brick chimney or leaving it exposed?

The removal of the ceiling really opened it up. It looks like you are doing a fun project. Good luck!!