With 58-Across, savory base for a canapé ... and the function of a black square in four puzzle rows?

All answers having "With 58-Across, savory base for a canapé ... and the function of a black square in four puzzle rows?" as clue:

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All answers having "With 58-Across, savory base for a canapé ... and the function of a black square in four puzzle rows?" as clue: