Bring the currants and sugar to a boil and cook for 5 minutes. Crush the currants with a wooden spoon and pass through strainer, pressing on them to release as much juice as possible.
Return the sweetened juice to a pan and reduce on medium heat, skimming occasionally to remove scum that rises to the top until 1 cup of liquid remains.
Transfer the coulis to an air tight container and refrigerate or freeze until needed. The natural pectin will thicken the coulis as it sits.