Identify how you’re using blackberries (fresh topping, baked filling, sauce, or jam-like swirl).
Choose the closest substitute from the list: 1:1 for raspberries/blueberries, then adjust down for sweeter, wetter options.
Apply the ratio and account for moisture: if the swap is frozen and your mixture looks thin, drain or simmer 2–3 minutes first.
Taste for sweetness and tartness; brighten with lemon juice for blueberries or add a pinch of salt to reduce overly sweet swaps.
For baked fillings, check thickness: simmer until it coats a spoon, then cool before assembling if your recipe requires set texture.