Bring 3 cups of the water and chocolate to simmer in medium saucepan on medium heat, stirring to melt chocolate. Stir in sugar and cardamom.
Place masa, remaining 1 cup water and blueberries in blender; cover. Blend on high speed until smooth.
Stir into chocolate mixture in saucepan. Cook on medium-low heat 8 to 11 minutes or until thickened, whisking occasionally. Sweeten to taste with additional sugar as desired. Serve hot.