Do not confuse the strength of black dye with blue dye. We use multiple dye colors mixed together to develop the black dye. As a result, the black dye is approximately 25% stronger than the blue dye. Furthermore, we start with a denser product so our black dye should tint a one acre pond 4ft. deep for 4 weeks. If you are not completely satisfied return the unused portion for a full refund including shipping.