Combine sugar, water, cardamom pods, and orange zest in a saucepan and cook over medium heat until it comes to a gentle boil and the sugar is dissolved. Remove from heat and heat it cool.
Whisk in the blood orange juice and strain the mixture into a bowl. Chill the mixture in the fridge for 1 hour.
When the mixture is cold, pour it into an ice cream maker and process according to the manufacturer's instructions.
Transfer the sorbet to an airtight glass or plastic freezer container. Cover tightly and freeze until the sorbet is firm, at least 4 hours.
To prepare the oranges, cut a slice off the top and bottom of each orange so they have a flat bottom to sit on. Use a sharp knife to cut the peel and remove as much of the pith from the oranges as you can. Slice the oranges into ¼ inch rounds and use the tip of your knife to pick out any seeds. Set the cut orange slices aside on a prep plate.
Scoop the sorbet into serving bowls and add orange slices before serving.