Blends are mixtures, GHS has a whole section on classification of mixtures. In absence of analytical data on the blended product, you need to consider the toxicity of the parent components and combine or assume the hazards of the most hazardous components. The producer of a product is responsible for communicating the hazards of their product; they do this through their SDS. If you change the composition of the product by blending it with other materials, you are responsible for communication the hazards of the new material.