You might try PM ladyonzlake, I think she has used one in the past...at least I think I remember her posting with that kind of filling.
i do a raspberry filling that rocks, you could easily do it with blackberries. when using fresh fruit, the juice usually seeps out and makes the cake soggy. the key is to thicken it so it stays put.
1 12oz bag of frozen berries
1/4 c sugar (if the fruit is very tart)
3 tablespoons cornstarch diluted in enough COLD water to disolve it.
thaw berries, puree. push through a sieve if you want to remove the seeds. put puree in a pot on medium heat on the stovetop. add sugar and starch liquid...stir constantly , and bring to a boil until well thickened. cool completely. this will become very thick and tastes very fresh. better than jam or preserves.
i like to add chambord to mine. this is a raspberry flavored liquor , and the reason i do it is to make it safe to leave out at room temperature for a day or two if i cannot refrigerate it because of fondant or other reasons. i was leary, but did some research and found that pastry chefs use this method frequently. if you do so, allow it to cool first, then add 2-3 tb and mix in, cover with plastic pressed against it to avoid a skin from forming, then chill until set (it will be like jam). yum!
This is the recipe I use for either blackberries or raspberries.
1/2 C sugar
2 T cornstarch
pinch nutmeg
1/4 C water
1 T unsalted butter
1 1/2 C berries (fresh or frozen)
In a small saucepan over moderate heat combine sugar, cornstarch, nutmeg. Gradually add the water and stir. Add the butter and berries and bring to a boil stirring gently. Boil for 2 minutes or until thickened.
This filling is so delicious especially made with fresh berries. You can also use canned berries. If using frozen let thaw and drain first.
Amy
I would be tempted to use blackberry jam putting a thin spread of BC down first, dam it (the layer) and fill it with jam.
The recipe I suggested cools thick! You never put on a "just cooked" filling anyway so once it is room temp or cool from the fridge it will be just perfect! I have had rave reviews using this as both a fresh raspberry and fresh just picked blackberry filling!
Amy
The good thing about this recipe is that the filling isn't that tart. It won't make your eyes roll into the back of your head. 
Do you charge for your cakes with the Polaner fruit? I've never tried the preserves. I see it in the store but have never tried it. What is the consistency and don't you have to buy a bunch of jars to fill the cake? What is your favorite preserve?
Amy
Raspberry or strawberry mixed with buttercream is my favorite.
I'm not crazy about fruit in cake myself but my mom sure is!
I've only had one order where the cakes were filled and it was for the church that I use their kitchen so I only charged cost. The consistancy isn't thin or horribly thick either. Just a thin layer is all you need. It depends on the size of the cake for how much you need or not. I did 12 4 in. round cakes and I had only used about 1 or 1 1/3 jar (can't queit remember) I think the homemade probly would taste better because most homemade items do. Funny thing is no one guesses it's just regular polaner jam in a cake 
I made a cake for a lady turning 80 (my mom and I threw her a suprise party since she NEVER had a birthday party---what a sorry family she has!) I made a hat cake and filled it with the blackberry jam in the bottom and a strawberry (or was it raspberry) mixed with b/c for the top. She loved it and swore it was from the blackberries she picked and gave me from her garden. I told her after the party and she laughed saying I fooled her.
I have a wedding cake ordered that the bride wants raspberry filling. I have been experimenting with different fillings and taking the cakes to my friend's office where about 30 people work and I have them "grade" the cake (they work for the school district). The only problem with the raspberry, cream cheese, and whipped cream filling is that the color was wrong. It was a pale purple and the bride wants deep red. Do you think I can add Wilton paste to it to change the color? Or what do you suggest? I think I will try another recipe and see if I can at least get it a deep purple, that would be better than the pale. Any suggestions would be appreciated.
This is the recipe I use. It is delicious and is a deep red. It is no other color than red! It is great to use with other berries as well!
This is the recipe I use for either blackberries or raspberries.
1/2 C sugar
2 T cornstarch
pinch nutmeg
1/4 C water
1 T unsalted butter
1 1/2 C berries (fresh or frozen)
In a small saucepan over moderate heat combine sugar, cornstarch, nutmeg. Gradually add the water and stir. Add the butter and berries and bring to a boil stirring gently. Boil for 2 minutes or until thickened.
This filling is so delicious especially made with fresh berries. You can also use canned berries. If using frozen let thaw and drain first.
Amy
You are welcome! I hope it works out well for you! Let us know!!!
Oh, just a note too. No need to add extra cornstarch to make sure it's thick. It would do nothing but add a "chalky" taste to the filling. This recipe is tried and true as is!!! It thickens upon sitting!
Amy
Cakemommy, I tried your raspberry filling and it was wonderful!
I just made raspberry filling for a lemon cake this evening that could easily be done with blackberries..... nothing fancy. sometimes simple is better!
5 lbs frozen berries
1 1/4 c sugar
3/4 c water
4 Tbsp cornstarch
1/8 tsp almond extract
1/4 c chambord
red food coloring
1. put all but cornstarch and chambord in pot and bring to boil, simmer 15-20 minutes.
2. mix cornstarch in chambord and then add to fruit mixture, stir, bring to boil and then let cool completely.
Turned out great. Next time I think I'm going to replace the 3/4 c water for chambord 
I just made raspberry filling for a lemon cake this evening that could easily be done with blackberries..... nothing fancy. sometimes simple is better!
5 lbs frozen berries
1 1/4 c sugar
3/4 c water
4 Tbsp cornstarch
1/8 tsp almond extract
1/4 c chambord
red food coloring
1. put all but cornstarch and chambord in pot and bring to boil, simmer 15-20 minutes.
2. mix cornstarch in chambord and then add to fruit mixture, stir, bring to boil and then let cool completely.
Turned out great. Next time I think I'm going to replace the 3/4 c water for chambord
I forgot to add.... after making the filling and preparing my cake (3 layer lemon cake w/ raspberry filling and lemonade cream cheese frosting) I had set it outside to cool as my fridge was full and the kitchen was rather warm so things were starting to slip and slip. Not usually a problem except for this time my hubby forgot I had done that and let the dog out! Needless to say we only had 1/2 of the cake to enjoy.
AI was looking for a good blackberry filling for my husbands birthday cake and I have to say Cakemommy this was a very easy recipie and very good, it thickened very nicely as it cooled and only took minutes! I am very pleased with the results, thank you!!
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