I'd like to make a blackberry pie for a friend (his fave kind of pie) but it's going to be well-nigh impossible to ferret out fresh blackberries this time of year. Any tips on using frozen ones? Do I let them thaw out first before using, or use them frozen? Do they cook up mushier than fresh, and thus should I add more thickener? I was thinking of using a Gourmet/Epicurious recipe that calls for a mix of cornstarch, sugar, and tapioca, which you mix into the berries and let stand for 20 minutes before putting into the pie shell and baking. Thanks!