A dirty blast is a drink used in the first subquest of Recipe for Disaster. It is made by adding Ashes to a player madeFruit blast, this requires a cooking level of 10. The Fruit blast obtained from The Dig Site quest can also be used. Its "Pre-made" counterpart obtained from Blurberry's Bar will NOT work to make it. Additionally, you cannot add the ashes until after the quest begins. Creating a Dirty blast gives 5 experience in Cooking.
The Dirty Blast, despite having a drink option, cannot be drunk nor emptied. It is useless after the quest.
The reason this disgusting drink is to be supplied by the player is not to make a wonderful dish. The Culinaromancer's recipe is in fact a powerful food-magic (as explained by the gypsy when you ask her what happened) and by preparing this recipe, the Lumbridge cook inadvertently revives the Culinaromancer.