These cracks are caused by rebar originally installed during construction to give strength to the chimney. Lack of maintenance, waterproofing, allowed moisture to penetrate to rebar and the expansion rusting rebar shatters chimney bricks. Extensive cracking due to lack of maintenance. Because chimney bricks are porous, water can soak through to rebar. Metal as it rusts, expands and blows up bricks.
2 issues here. In an attempt to remodel a fireplace, the lintel was removed making the fireplace opening taller. This might be aesthetically pleasing, but now the fireplace has been damaged and likely will not draft properly. Nearly impossible to repair.
The black streaking shows evidence of cracks in firebox allowing smoke and hot gasses to potentially get into combustibles.