Remove the razor from the blade box knife casing. Carefully trace the shape of the razor on the poster board. Cut around the tracing with the scissors. Make several more copies of the razor on the poster board. Cut out the tracings. Glue the tracings together to make a sturdy cardboard blade. Spray paint the blade sliver.
Punch a hole in the cardboard blade using the end of the scissors. Insert the cardboard blade into the box knife case. Paint the box knife casing black.
Hold the open side of the knife down when on stage to keep the keep the audience from seeing it. Use the slide button to retract the knife blade and re-extend the blade as needed. Make fake blood using corn syrup, water, and red and yellow food coloring for a more realistic stab wound.