How to fix the consistency of blueberry jam?
Jam is fruit, sugar, and pectin. You added more fruit but all that liquid in the fruit wasn't gelled with the pectin and sugar.

You could try adding sugar and heating the jam to get the pectin to gel with the new juice. This could work but is likely to be error prone. Pectin gelling is tricky and it might not re-gel.

I would recommend adding some sugar, boil it for a bit, and use it as delicious syrup.

In the future, if you have fruit and want jam, just buy some pectin and make your jam from scratch. It's easy to do.