A dirty blast is a drink used in the first subquest of Recipe for Disaster. It is made by adding ashes to a fruit blast. Its premade counterpart obtained from Blurberry's Bar will not work. Additionally, you cannot add the ashes until after the quest begins. Creating a dirty blast gives 5 Cooking experience.
The dirty blast, despite having a drink option, cannot be drunk nor emptied. It is useless after the quest. The fastest way to make it is going to the Grand Exchange and buying a fruit blast then add the ashes.
This drink was part of a spell trapping the Culinaromancer in another dimension. A century later, the descendent of the spell creator mistook the spell for a grand banquet and requested aid in obtaining the ingredients, including the dirty blast. He recreated the spell, accidentally freeing the Culinaromancer from his prison.