I never use fingers for black key glissandi - just an open palm lightly brushing over the tops of the keys (NOT past the escapement level). Or a small stuffed animal.
Yea, I can see the kids taking "fluffy" to their audition

It would be fun in the lesson though, just kidding you.
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Then let us all do what is right, strive with all our might toward the unattainable, develop as fully as we can the gifts God has given us,and never stop learning." ~ Ludwig van Beethoven