In a 1 quart resealable plastic bag, add beef and ponzu sauce. Seal bag, eliminating as much air as possible. Refrigerate 4-8 hours.
Drain beef and discard marinade.
In a large non-stick pan over medium-high heat, cook beef with canola oil in batches, stirring often until cooked through and liquid has evaporated. Transfer to a plate and cover until all meat is cooked.
Add water to a medium saucepan. Bring to a simmer. Stir in rice, then cover and cook on low for 20-30 minutes or until rice is tender. Add more water if rice is not cooked through. Transfer rice to a container, cover, and refrigerate.
Spray a small 1/4 cup mold with non-stick spray. Fill half full with cooked black rice. Remove from mold by turning over onto a baking sheet covered with non-stick foil. Place one or two pieces of cooked beef on each cake and transfer with a small spatula to serving trays.