Wash oranges well. Use a peeler to remove all the zest in long strips. It's okay if some of the white pith comes with it.

Transfer the peels to a saucepan and add 6 cups of cold water. Bring to a simmer over high heat. Reduce heat to medium-low to low and simmer until the peels are soft and tender, 45 minutes to 1 hour.

While that cooks, cut oranges in half and juice them into a large measuring cup to equal 1 cup. Add 1/2 cup cold water and set aside.

Remove peels from heat and drain off the water. Transfer peels to a cutting board, and when cool enough to handle, slice the zest into very thin strips. Transfer into the blood orange juice.

Pour zest-juice mixture into the saucepan along with lemon juice and sugar. Bring to a simmer over medium-high heat. Reduce heat to medium-low and cook, stirring occasionally, until the mixture reduces and thickens slightly, 30 to 40 minutes. A candy thermometer should read 225°F (107°C).

Inspect jam jars for cracks or rust. Immerse in simmering water until juice mixture is ready. Wash new lids and rings in warm soapy water.