well you can do a few things, you can get more blackberry concentrate and add it into the wine which will increase flavor, but it will also cut the ABV a little bit depending on how much you need to add to get the flavoring you want.
or you can search online and find blackberry extract and do some bench testing to get the flavor you want this will also cut ABV some but it would be far less than adding juice and you wouldnt get the extra sugar as you would from the juice.
or you can just drop blackberries in the secondary and let it sit.
whatever you do make sure you add k-meta and sorbate prior to so fermentation doesn't start up again. As for the insinglass make sure your wine is degassed, and toss it in you should be good to go from there.
one word of warning though, your wine is young so the alcohol will be more pronounced right now, as this ages the berry flavor will come forward more and the alcohol edges will smooth out so try not to over sweeten.
