Line an 8x8-inch square baking pan with parchment paper, leaving an overhang on the sides. Lightly grease.

In a heatproof bowl, combine 2 cups semi-sweet chocolate chips, 7 oz sweetened condensed milk, 1/4 cup black cocoa powder, 1 tbsp butter, and 1/4 tsp salt. Microwave in 30-second intervals at 50% power, stirring until a thick, black ribbon forms.

Stir 1/2 tsp vanilla into the black chocolate mixture. Pour into the prepared pan and press firmly into an even, dense layer.

In a clean heatproof bowl, combine 2 cups white chocolate chips, 7 oz sweetened condensed milk, 1 tbsp butter, and 1/4 tsp salt. Microwave at 50% power in 20-second bursts until smooth. Stir in 1/2 tsp vanilla.

Fold 1 tsp red gel food coloring into the melted white chocolate until vibrant blood-red. Pour directly over the black chocolate base, smoothing it out.

Dot the surface with 2 tbsp cherry preserves. Use a toothpick to drag through the preserves and the red chocolate, creating a glossy blood-red swirled effect.

Refrigerate for at least 2 hours until firm. Lift out using the parchment overhang and use a hot knife to slice into dense, square-cut pieces.