Might be a question for a stone mason ... I will give this a bump for you !! Personally I'd guess - something easy to clean - nonporous - maybe something that's thin that's then set on wood to allow to be float fixed ... Thin slate might work !?! But I'm not an expert in this field - I'm a planning geek ! Hope someone in the know chimes in to offer assistance!! ;-)

How are you with painting?? If the surface smooth enough it could work well, then you can have the designs that you want at the cost of two pots of paint and some masking tape! A little time consuming perhaps but very cheap. My wife, has painted concrete several times for clients. She's painted concrete on a chimney stack to mimic old reclaim bricks and mortar where someone did a patch job, and even a circular pond on a patio ( fake obviously ). Givei it a go.

Current state of fireplace and the look we are going for (but black).

How strange! I wonder why they used so much cement under it? I would remove the hearth (assuming it's been fitted around the mantel and the mantel isn't sitting on top of the hearth). You could then chisel out the cement. I personally think if you are keeping the fire, keep the hearth but lower it., you are not losing much floor space and the fireplace may look 'odd' without it.