A dirty blast is a drink used in the first subquest of Recipe for Disaster. It is made by adding ashes to a player made fruit blast, requiring 10 Cooking. 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 Cooking experience.
It is needed along with a Rotten tomato, an Eye of newt, and a Greenman's ale for the Recipe for Disaster quest.
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.