is 56% chocolate dark enough for flourless chocolate cake?

Are you more of a milk-choc or semisweet girl than dark choc? The recipe will work wih whichever % dark choc. you have. And if you have on hand some Unsweetened or baking choc or cocoa powder, you could combine those with your 56% to get it darker (get specific equivalents from Joy of Cooking or Alice Medrich's books.)

Often recipes call for bittersweet chocolate or with minimum 70% cocoa solids, but it would probably work with 56%.

Depends on the recipe as well as your particular taste.

Good luck.