A blend is a formulation, and specifying it as though it were a single product is the source of most disputes in the category.
Specify each component to its own grade. A blend is only as good as its weakest component, and downgrading one component is the easiest invisible cost reduction available to a supplier.
Fix cut size per component with a tolerance. Cut size is a cooking-time parameter, not a cosmetic one. A dense root cut to the same nominal dimension as a soft vegetable will still be firm when the soft component has collapsed.
State the ratio with a tolerance and a sampling protocol. Components of different density and particle size segregate in transit, so a sample drawn from one layer of a settled carton will fail for the wrong reason. Specify full-depth sampling, a sample mass, and measurement on a thawed and separated sample.
Require per-component blanch validation. A single treatment applied to a mixed stream over-cooks the thin components and under-cooks the dense ones. Under-blanching leaves residual enzyme activity that produces off-flavours during frozen storage; over-blanching costs texture irreversibly.
Write a substitution clause before the first substitution. Every mix programme eventually substitutes a component because of a crop failure or a price move. Requiring notification and approval in advance is the difference between a managed change and a discovery.
Date from the oldest component. A pallet of mix routinely contains more than one crop year. Ask for component-level lot traceability rather than a single blend lot number, because otherwise a recall on one component takes every blend containing it.
Finally, hold a sealed physical reference sample renewed each season. It is the cheapest control in the list and settles more arguments than the rest combined.