I did two different dark beers, a rauchbier and a imperial stout, and noticed a grape aroma and flavor in them. The fermentables that there're in common are: pilsen, flaked oats, caramalt and black malt. My intuition says the black malt is the source of grape's flavor, but I'm not sure. While in the rauchbier (fermented at 18C, 5% of black malt) this flavor was too unpleasant, in the imperial stout (fermented at 16C, 1% black malt) it was smoother and didn't dominate. Apart from the difference in the temperature (higher producing more esters, i.e., fruity flavors), where are this grape flavor coming from? Is it coming from the black malt indeed?
Black malts usually contribute a roasty flavor and color enhancement. As far as I know, they shouldn't impart a fruity flavor. Caramel malts, on the other hand, do. Check your caramalts.
Common flavor descriptions for roasted caramel malts between 80L and 120L are
"raisin, plum, prunes, burnt sugar".