To keep cement off the floor surface, position tar paper on the floor past where the hearth will go and secure it with electrical tape.
Tape plastic sheeting on top of this to further protect floors.
Place the blocks into position in front of the fireplace.
Mix concrete, then fill each block with concrete to about 3" deep. Push the concrete down so it will settle evenly.
After the concrete has dried, use a level to make sure the hearth is still straight.
Spread mortar over the entire hearth, top and sides, then mix plaster and color pigments to the desired tint (Image 1).
Test the color by placing it on a sample board and letting it dry in the sun.
If the color is the desired tint after drying, use a trowel to smear the plaster onto the hearth surface (Image 2).
Create the look of bricks and mortar by using a tongue depressor to cut through red plaster and expose the gray concrete.
Shape some of the corners to give the "bricks" the look of being cut with a saw.
Dab a little gray and black cement onto the bricks to create an antiqued appearance and help the new hearth blend with the old fireplace.