I installed 20+ tvs on the wall for family and friends. I've done 3 over fireplaces. It really isn't much different, get a good masonry bit and ask the guys at Lowes or Home Depot for special anchors. Should take less then 45 min to get it done. If you hire out, you're talkikg $300-$600 for a job you could seriously do in under 45 minutes.
As a contractor, I would never use them, but if I were a homeowner and didn't know anybody, then I would give them a try.
Otherwise, ask around and start with an electrician if you don't want a TV specialist.