Interesting question. I'm going to start with the last part of your question. Fruit is a plant's way of getting its seeds somewhere. It's sort of a bribe to an animal to carry the seeds to a new place where they won't compete with the parent tree. Here's how it works. Inside the orange blossom (flower) are a bunch of egg cells that get fertilized by pollen. The eggs are then baby seeds.

The container that the eggs were in (the ovary) keeps getting bigger. The plant pumps water, sugar, and other things into the ovary, which gets bigger and sweeter. A sweet, juicy fruit attracts animals, who eat the fruit and usually swallow the seeds. Then they move on. When the animal poops later, the seeds have a nice bunch of fertilizer to grow in.

So you might be thinking that juice gets injected into the fully formed orange, but the whole fruit develops over weeks. Plants have tubes inside that are sort of like blood vessels. They move things like sugar and water around the tree. The water comes up from the roots. The sugars come from leaves. In the leaves, cells do photosynthesis, which takes carbon dioxide from the air and water to make sugar. Sunlight provides the energy to power making the sugar. So a lot of the orange is water, but the rest came from the air! The carbon dioxide comes into the leaves through tiny holes.

Blood oranges are just a type of orange with a lot of a particular color, or pigment. Lots of fruits turn color to signal to animals that they are ripe. They only do that when the seeds are mature. Before that, they usually taste bad and may even be poisonous. Of course, the tree doesn't know any of this. Trees that get the timing right are more successful than trees that don't. Chlorophyll is another pigment. It is essential in photosynthesis.

Why do you think oranges have such a thick rind (skin)?

Maybe that's more than you wanted to know, but thanks for asking.

Blood oranges naturally contain a chemical called "anthocyanin." This chemical occurs in a lot of other fruits and vegetables, and usually makes them red, purple, or blue in color. Blueberries, grapes, red cabbage, and blood oranges are all examples of foods that have a lot of anthocyanin. However, regular oranges (and most other citrus fruit) don't contain very much anthocyanin- this is why they are much lighter in color.

The red coloration is due to a set of compounds (basically, large molecules) called anthocyanins, which are part of a family of polyphenol pigments. Like all pigments, they change the color of light by absorbing specific wavelengths and reflecting the rest. These molecules are naturally produced by the fruit, specifically when exposed to low nighttime temperatures during ripening.

Anthocyanins are produced by a process called the phenylpropanoid pathway. This means that they are assembled from the products of two particular cellular paths, one of which produces the amino acid phenylalanine and the other which produces malonyl coenzyme A. (There is no simple description of that pathway, and the linked page has many esoteric chemical names and words. An even less-accessible description is in this journal article, which biochemists might like.)

The blood orange gets its red colored juice from pigments called anthocyanins. Anthocyanins are responsible for coloring both flowers and fruits. These colors are what help attract pollinators to flowers and animals to edible fruits to help disperse seeds. In the blood orange, the specific anthocyanin pigment inside is called chrysanthemin. Chrysanthemin is also found in raspberries, soybean shells, plums, and peaches.

Blood oranges are the only citrus fruit to have this red pigment. Blood oranges develop their special red color when growing through nighttime cold temperatures. You can also help blood oranges to continue to turn red inside by storing them in the refrigerator after picking them.

Blood orange grows with red juice inside, just like regular oranges grow with orange juice inside. The red color comes from biological molecules called anthocyanins. Anthocyanins are common in lots of flowers and fruits, but they're unusual in citrus fruits like oranges. From this link: “The blood orange is a variety of orange (Citrus × sinensis) with crimson, almost blood-colored flesh. The distinctive dark flesh color is due to the presence of anthocyanins, a family of polyphenol pigments common to many flowers and fruit, but uncommon in citrus fruits. “

Hello Amanda! Blood oranges get their color from a special dye or pigment called anthocyanin. This dye is in the juice of the fruit, which is why it has a red color.