Zest one or two of the oranges before juicing; set zest aside for later. Juice oranges, then strain through a fine mesh sieve to remove any pith bits or pieces of flesh. You'll need 1 cup of freshly squeezed juice to start.
Simmer orange juice in a medium saucepan set over medium-high heat, stirring occasionally, until reduced by half, about 10 to 15 minutes. You should have 1/2 cup of juice. Add the citric acid and stir until dissolved. (The reduced juice can be made a day ahead of time; transfer to a jar or other container and refrigerate until ready to use.)
To make the curd layer, prepare a double boiler by filling a saucepan with an inch or two of water and bring to a simmer. Set a medium heat-proof bowl (I prefer pyrex over thin stainless for this since it conducts heat more slowly and evenly). The bottom of the bowl should sit over, but not touch the water.
Whisk together eggs, egg yolks, sugar, and orange zest in the bowl. Whisk in 1/2 cup of your reduced orange juice. Continue to cook, whisking regularly and adjusting the heat as necessary so the water remains at a gentle simmer, until the curd noticeably thickens. At this point it should read approximately 165 degrees F on an instant-read thermometer. Remove from heat and set aside. If you notice some chunks in your curd (which happens if the temperature gets too hot too quickly), just press it through a fine mesh sieve to remove any stray bits of egg.
Preheat oven to 350 degrees F. Lightly butter and line an 8-by-8-inch square baking pan with parchment (the butter will keep the parchment from moving when you spread in the batter). Lightly butter parchment.
To make brownie layer, melt butter in a small saucepan set over medium heat (you can also reuse your double boiler setup too if you have a second bowl that works). When it is completely melted and starts to bubble, remove from heat. Add sugar and stir until smooth and paste-like. Whisk in oil.
Add egg and egg yolk and whisk vigorously for a full minute. Whisk in vanilla.
Add flour, cocoa powder and salt and fold in until just incorporated.
Pour into prepared baking pan. Rather than dumping it all in the middle, it’ll be easier to spread the thick batter if you drop dollops of batter around the pan. Using a thin offset spatula, spread batter into an even layer.
Bake for 15 to 17 minutes until the top appears uniformly set, though a toothpick tester will still be quite gooey.
Remove from oven and pour orange curd on top, carefully spreading into an even layer (since the brownie is still somewhat undercooked, you need to be gentle here so as not to pull up the chocolate batter).
Return to oven for 5 to 7 minutes or until curd layer appears evenly set. Remove from heat and place pan on a wire rack to cool for about an hour, then refrigerate for at least an hour or overnight until fully set.
Lift brownies out of pan using parchment as handles. Cut into 16 squares. For the cleanest cuts, run a large chef's knife under hot water to warm the blade, then wipe with a tiny bit of oil or butter. Clean off blade (and re-heat/re-oil as needed) between each cut.
Brownies will keep refrigerated in an airtight container for up to 3 days.
Notes
The color of blood oranges can vary greatly, and the color of the final curd may range from a pale mauve to a deep maroon. Feel free to add a few drops of red food coloring if you'd like to enhance the natural color.