Mix the blueberries, lemon juice, cinnamon and salt with 4 T. of the sugar in a saucepan. Bring it to a boil and simmer, stirring for about 10 minutes to soften the blueberries.
Let it cool a few minutes, then put into a food processor and puree. Filter it through a fine sieve to get out as much liquid as possible. Refrigerate the puree for about an hour.
Beat the cream on high speed in a mixer with the remaining 3 T of sugar. Whip until soft peaks form. Fold the puree gently into the cream until the color mixes and it turns purple. Refrigerate for about 1/2 an hour before using to fill the cake.