I started out making a blackberry-Cabernet wine, adding if I remember 6 jars of jam to 3 gallons of water, x amount of sugar to a SG of 1.090 or about 12 % ABV. I used EC1118 yeast and low and behold I ended up with stuck fermentations every time, just about the same SG as you. If I recall correctly there is something in the jam (chemical) that interferes with the fermentation process. I added yeast nutrient, warmed the wine and coaxed it to finish. It took a couple of weeks, never went totally dry, but blended with a Cab, it was pretty good. Next time don’t use jam, buy an Alexander product of blackberry concentrate. Much more predictable and satisfying.