Mix together the light and dark rum, banana and blackberry liqueur, orange and pineapple juice, and grenadine in a mixing glass (or a cocktail shaker).
Add some ice mixing glass, and stir (or shake) it thoroughly until it's well blended and chilled.
Fill a tall glass with crushed ice, strain the cocktail mix into the glass. Add more ice if needed. Garnish with some fruit, like an orange slice, pineapple chunk, or cherry.