You could also do a black heart for the black marble.
As for the smiley, weren't there some blueish smiley-like actors that you could bounce around? Especially since there is a wide variety of smileys, I'd keep them for some later game gimmick.
Like having to collect different smileys or something like that. Having to pick them up somewhere and place them on special locations to activate something, whatever. Also smileys can serve as some kind of indicator (frowning = bad, happy = good). Maybe sometimes as hint to get an easy mode for some levels.