Structure for mounting TV above fireplace
If I were you I would frame in a 2x10 flush with the tile roughly a little longer than the TV's mounting bracket.  It would be hidden for the most part behind the TV and would be secured to the wall studs for extra strength. It can be painted too.

As a side note, it's important to use pressure treated lumber if building on top of an existing fireplace. 

Our TV is mounted above the fireplace.  It uses a special hanging adapter made by the TV company that uses a steal cable so it literally hangs like a picture.  I used two 5/8 lags with washers to hang it, but could have opted for a normal TV hanging mount. Though it would have probably not have been flush.