Prepare canner, jars and lids. See The Ball Complete Book of Home Preserving for guidance. Lay out a clean towel, along with a small bowl of white vinegar (to wipe off rims) and your canning supplies on a clean counter.

Place a small plate into your freezer.

Use a vegetable peeler to peel off the thinnest layers from the blood oranges and then slice them in very thin slivers.

Peel the oranges and segment them. As much as possible, remove the membranes from the segments.

Dice the plums into 1/2" to 1" cubes

Put plums, orange segments and orange peel slivers into jam pot (preserving pan) along with sugar.

Bring to a simmer, stirring frequently, for 10 minutes.

Pour the mixture into a mesh colander over a pot, stirring gently to drain off any liquid into the pot.

Return the liquid to heat and bring to a boil over high heat.  Boil while stirring frequently until liquid is thickened and reduced, which should take about 10 minutes

Add the fruit back to the pot, along with the lemon juice, and bring mixture to a simmer, stirring frequently until preserves pass the 'freezer test' 

(Freezer Test:  Dollop a small amount of preserves on chilled plate, and put it in the freezing compartment of a refrigerator for a few minutes. If, after a few minutes when you push on the dollop with your finger, it wrinkles a bit, it should be done. During this test, the rest of the jelly mixture should be removed from the heat.)

Remove from heat.

Spoon your jam into hot jars, leaving 1/4 inch headspace if using Ball/Kerr or Hexagon jars or 1/2 inch if using Weck jars

Remove air bubbles and wipe the rim with white vinegar.

Center lid on jar and screw band down until resistance is met, then increase to fingertip tight. Or, if using Weck Jar, place rubber gasket, glass lid and metal clips on the jar.

Place jars in canner, making sure they are completely covered with water by at least one inch.

Cover pot and bring to a boil

Boil for 10 full minutes, or adjust for altitude via Ball Complete Book of Home Preserving. The time to bring your pot to a boil should not be counted in the minute boil time.

Turn heat off, remove lid and let sit for 5 minutes

Remove your jars from the pot and let them sit, undisturbed for 24 hours. Your Ball/Kerr jar lid centers should pop down when sealed and the orange tab on the Weck jars will slightly curve down.