Actual costs:
1x1x1 = 2 marbles (used as standard for comparing the other blocks)
2x1x1 = 4 marbles
2x2x1 = 4 marbles (It should logicly cost twice as much)
2x2x2 = 8 marbles (It should logicly cost twice as much)

I do agree with the OP though, If the piece he/she is looking for is already in the game, adding it to the build menu should be possible.
We do have 4x2 for regular stone (cost 6 stone) , so a 4x2 for black marble wouldn't be out of place. Or as the OP asked for, a 2x2 with a thickness of 1.

After all, it only adds to the game, it doesn't subtract nor nerf , so i would welcome it.