Blends are two or more compounds supplied together in a single vial at a fixed ratio. Because the components are not measured separately, a blend is treated here as its own subject rather than as a sum of its parts.
A blend arrives pre-mixed, so the ratio between its components is fixed by the vial rather than chosen at the bench. Compounds covered on their own are under Single Peptides, and compounds kept in separate vials but studied together are under Peptide Stacks.