Wash and scrub the blood oranges with warm soapy water to remove any wax, then dry the oranges.
Put sugar in a bowl and zest the blood oranges directly into the bowl. Use your fingers to rub the zest into the sugar.
In another bowl, use a citrus reamer to extract juice from the blood oranges. Strain and set aside.
Soak gelatin sheets in a bowl of cold water for 10 minutes to let them bloom.
Combine cream, labneh, sugar, and blood orange zest in a small saucepan over medium heat and bring to a simmer. Remove saucepan from heat and set aside.
Whisk in the blood orange juice until well combined
Lift the gelatin sheets from the cold water and wring gently to remove excess water.
Add the gelatin sheets to the warm cream mixture and stir until they melt completely.
Strain the blood orange mixture through a fine sieve and let it cool for 30 minutes before pouring it into the tart shell.
Refrigerate for at least 4 hours or overnight for the filling to set.