Using a serrated bread knife in a gentle sawing motion, trim and discard crusts from bread slices. Cut bread slices into 8 (2½×1½-inch) rectangles.
In a small bowl, stir together mayonnaise, blue cheese, parsley, and pepper until combined. Spread a layer of mayonnaise mixture onto bread rectangles.
Using a sharp knife, cut cucumber in half lengthwise. Using a mandoline, cut 32 very thin lengthwise slices from cucumber halves.
Arrange 4 cucumber slices width-wise in a shingled, parallel fashion on mayonnaise side of each bread rectangle, trimming cucumber slices to fit bread rectangle.
Garnish each canapé with a parsley leaf, if desired. Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate for up to an hour before serving.
Recipe by TeaTime Magazine at https://teatimemagazine.com/cucumber-blue-cheese-canapes-recipe/