Hi Jean:-) I made Raspberry Coulis to serve with a Chocholate Decadence Cake. I froze the leftover Coulis and it was good for a long time. It's great over ice cream or Pound Cake as well. I imagine it might work for pancakes, too. I see no reason that it couldn't be made with blackberry juice.

RASPBERRY COULIS
Source: The Cake Bible by Rose Levy Beranbaum

2(12 ounce) bags, unsweetened, frozen raspberries (680 grams)
2 tsp lemon juice (10 grams)
2/3 c sugar (132 grams)

In a strainer suspended over a deep bowl thaw the raspberries completely. (Will take several hours). Press the berries to force out all the juice. There should be 1 cup.

In a saucepan, boil the juice until it is reduced to 1/4 cup.

Puree the berries and sieve them through a food mill fitted with the fine disc. Or use a fine strainer to remove all the seeds. You should have 1 liquid cup puree. Stir in the raspberry syrup and lemon juice. To make a lightly sweetened sauce, measure again. There should be 1 1/3 liquid cups. If you have less, add less sugar. The correct amount of sugar is half the volume of the puree. (To 1 cup, add 1/2 cup sugar.) Stir until sugar dissolves. This may be stored 10 days in the refrigerator, 1 year in the freezer. The puree can be thawed, briefly, and refrozen several times with no ill effect.

Tip: If using fresh berries, you will need 1 1/2 pounds or 1 1/2 quarts. In order to make them exude their juices, they must be frozen and thawed to break down the cell membranes.

This is a lot of work but delicious & worth the effort!