Line a baking tray (sheet) with silicone (baking) paper. Put the blackcurrant purée and invert sugar in a saucepan, bring to the boil and then leave to cool to 65-70°C (149-158°F).
Melt the chocolate over a bain-marie (water bath) until it reaches 45°C (113°F). Gradually add the fruit mixture to the chocolate, mixing continuously to form an emulsion. Add the bu