The filling in Nigella's Salted Chocolate Tart still looks a deep brown colour after making. How is this when the chocolate is mixed with so much milk and cream? Sorry to ask but I am not an experienced cook.
The filling uses dark (bittersweet) chocolate that is mixed with milk and cream. The colour of chocolate melted into milk will be paler, but it will not turn quite the same colour as milk chocolate. Milk chocolate is usually made with between 20% and 37% cocoa solids and the rest is sugar, extra cocoa butter, milk solids and some vegetable fat and this contributes to the colour. Milk and cream don't have the same fat content and are actually made up mostly of water (milk is 80-85% and double cream is approximately 55%), which affects the colour less. In addition, the Salted Chocolate Tart filling contains cocoa powder, vanilla paste and instant espresso powder. These provide depth of flavour but also help to give the filling a dark hue.