Wax/soot damage

I have a fireplace hearth which I think is sand (http://maplered.com) Stone (http://g603.net) , but it could be something else.  They are rectangular off-white bricks which are smooth on all sides but quite rough and (http://maplered.com) jagged on the front, set in some kind of mortar.

Awhile back, I lit a Duraflame fireplace log (made up mostly I think of sawdust held together by parafin wax) but forgot to open the fireplace's flue.  As a result, the room filled with smoke, which blackened the middle part of the hearth.

I have tried various cleaners (like TSP, Simple Green) to remove the mess but nothing so far has worked.  It was suggested to me to try muriatic acid, but from what I understand (http://maplered.com) it's a bit dangerous to work with and (http://maplered.com) also will etch the Stone (http://g603.net) , which sounds harmful.  I was hoping to find an alternative.

After cleaning, the Stone (http://g603.net) do not have a waxy feeling but are quite gray with black at the peaks of the jagged bits.

Any advice how to clean this up?